Optics: notes and MCQs for State PSC prelims

Optics is part of Physics. Below are notes on 12 sub-topics, followed by practice MCQs from State PSC and UPSC papers.

Fiber Optics: Principles and applications

Optical fiber communication is based on the principle of Total Internal Reflection (TIR) of light.
In optical fiber communication, light acts as the carrier wave.
Light rays travel through an optical fiber by undergoing successive total internal reflections, allowing them to follow a zigzag path and transmit signals over long distances.
Applications of Total Internal Reflection include:

Optical fiber communication.
Endoscopy, a technique used to explore internal body parts.

A photoelectric cell is a device that converts light energy into electric energy.
TV remote control units typically use infrared (IR) light waves to operate a TV set.
Infrared light is commonly used in:

Night vision apparatus.
Detection of forged documents.
Greenhouses to keep plants warm.

Holography

Holography: Technique for recording and reconstructing three-dimensional (3D) images.
Key feature: Records both amplitude and phase information of light waves.
Light source: Requires coherent light, specifically a laser beam, for recording.

Interference: Young's double-slit experiment, thin films, Newton's Rings

Interference of Light: Superposition of two or more coherent light waves resulting in a stable pattern of varying intensity (bright and dark fringes).
Conditions for sustained interference: Coherent sources (constant phase difference), monochromatic light, sources close together, small source-screen distance.
Young's Double-Slit Experiment (YDSE):

Setup: Two narrow, parallel slits (S1, S2) illuminated by a monochromatic source, acting as coherent sources.
Constructive Interference (Bright Fringes): Path difference, Δx = nλ (where n = 0, 1, 2, ...).
Destructive Interference (Dark Fringes): Path difference, Δx = (n + 1/2)λ (where n = 0, 1, 2, ...).
Fringe Width (β or w): Separation between two consecutive bright or dark fringes. β = λD/d.
Where: λ = wavelength of light, D = distance from slits to screen, d = distance between the two slits.
In Fresnel's Biprism (a YDSE variant), the wavelength (λ) depends on the fringe width (β), the distance between the two coherent sources (d), and the distance between the sources and the screen/eyepiece (D), as per β = λD/d.

Interference in Thin Films:

Phenomenon: Produces beautiful colours when illuminated by white light due to interference of light waves reflected from the top and bottom surfaces of the film.
Examples: Soap bubbles, oil films on water (causes brilliant colours on rainy days and in daylight).
Mechanism: Path difference between reflected rays depends on film thickness, refractive index, and angle of incidence. A phase change of π (or λ/2) occurs when light reflects from an interface with a denser medium.
Conditions for Constructive/Destructive Interference (for reflected light from an air-film-air system like soap bubble):

Constructive (Bright): 2µt cos r = (n + 1/2)λ
Destructive (Dark): 2µt cos r = nλ
Where: µ = refractive index of film, t = thickness, r = angle of refraction, n = 0, 1, 2...

Newton's Rings:

Specific case of interference in a thin air film formed between a plano-convex lens and a plane glass plate.
Pattern: Concentric bright and dark rings, with a dark spot at the center (due to a path difference of zero and a phase change on reflection).
Radius of Dark Rings (rn): rn² = nRλ (where n = 0, 1, 2..., R = radius of curvature of the lens).
Radius of Bright Rings (rn): rn² = (2n - 1)Rλ/2 (where n = 1, 2, 3...).

Additive Color Mixing: When red, blue, and green light (primary additive colours) beams fall on the same spot, the resultant colour is white.

Laser: Principles and applications

LASER stands for Light Amplification by Stimulated Emission of Radiation.
LASER operation is primarily based on Stimulated Emission of Radiation.
Key properties of a LASER beam include:

High Monochromaticity (single wavelength).
High Coherence (waves maintain constant phase relationship).
High Intensity (highly concentrated energy).
Low Divergence (highly collimated, narrow beam).

The property High Optical Activity is not related to LASERs.
The first working laser, constructed in 1960 by Theodore Maiman, used a Ruby Rod as its active medium.
LASER applications include:

Measuring distances to distant objects (e.g., moon) due to high coherence and low divergence.
Used in CD (Compact Disc) audio players for reproducing sound.
Industrial cutting and welding.
Medical procedures.

LED stands for Light Emitting Diode.
The working principle of an LED system is Electroluminescence, converting electrical energy into light energy.
Regarding street-lighting:

Sodium lamps produce light in 360 degrees and have an almost monochromatic visible light spectrum.
LED lamps produce directional light, offer significant colour advantages, and generally have a longer lifespan than sodium lamps.

Solar cells, also known as Photovoltaic cells, convert light energy into electrical energy.
Photoelectric effect facts:

Albert Einstein received the Nobel Prize in Physics for explaining the photoelectric effect.
For each metal, there is a threshold frequency (v₀) below which the effect is not observed.
At a frequency v > v₀, the kinetic energy of ejected electrons increases with increasing frequency of incident light.
The number of electrons ejected is proportional to the intensity of incident light.

The Raman effect is used in the study of Molecular Energy (e.g., vibrational and rotational modes) and in intelligence devices for material analysis.
Colours on the screen of a TV are produced by mixing Red, Green, and Blue (RGB).

Optical Instruments: microscopes, telescopes, human eye

Human Eye:
Functions like a camera, forming a real, inverted image on the retina.
Image is interpreted by the brain as erect.
Lens: Convex (converging), with variable focal length (accommodation).
Focal Length Control: Ciliary muscles adjust the curvature of the eye lens.
Pupil: Regulates the amount of light entering the eye; decreases in size in bright light, increases in dim light.
Retina: Light-sensitive screen containing rods (for dim light) and cones (for colour and bright light). It converts light into electrical signals sent to the brain.
Refractive Index: Lens has the highest refractive index among eye parts.
Least Distance of Distinct Vision (LDDV) / Near Point: 25 cm for a normal eye.
Far Point: Infinity for a normal eye.
Visible Spectrum: Wavelengths from approximately 380 nm (violet) to 760 nm (red).
Colour Sensitivity: Most sensitive to colours between yellow and green.
Visual Acuity: Ability to discern shapes and details of objects.
Binocular Vision: Using both eyes helps in judging the relative positions (depth) of objects accurately.
Space Appearance: Outer space appears black to an astronaut due to the absence of an atmosphere to scatter sunlight.
Common Eye Defects and Correction:
Myopia (Nearsightedness):
Symptom: Can see near objects clearly but distant objects appear blurred.
Cause: Image of distant objects is focused before the retina, due to either excessive curvature (increased power) of the eye lens or elongation of the eyeball.
Correction: Concave (diverging) lens of suitable power.
Lens Power Calculation: If the far point is 'x' meters, the power of the corrective lens is -1/ x Dioptre. (e.g., for far point 2m, P = -0.5 D).
Hypermetropia (Farsightedness):
Symptom: Can see far objects distinctly but near objects appear blurred.
Cause: Image of near objects is focused behind the retina, due to either reduced curvature (decreased power) of the eye lens or shortening of the eyeball.
Correction: Convex (converging) lens of suitable power.
Astigmatism:
Cause: Imperfectly spherical cornea.
Correction: Cylindrical lens.
Microscopes:
Function: Used to observe magnified images of small objects.
Magnification (M): Ratio of observed size of specimen to actual size of specimen.
Compound Microscope: Consists of two convex lenses:
Objective Lens: Short focal length, forms a real, inverted, magnified intermediate image.
Eyepiece Lens (Ocular): Short focal length (typically longer than objective's focal length), acts as a simple magnifier, producing a final virtual, inverted, and highly magnified image.
Magnification: Increases with a decrease in the focal length of both the objective and the eyepiece. It also depends on the length of the microscope tube.
Components (e.g.): Objective lens, eyepiece, stage, clips, mirror (illuminator), focusing knobs. Retina is not a part of a microscope.
Smallest Object: Optical microscopes can clearly resolve objects in the micrometer range.
Telescopes:
Function: Used to observe magnified images of distant objects.
Types:
Astronomical Telescope: Uses a convex objective lens (long focal length, large aperture) and a convex eyepiece lens (short focal length). Forms a final virtual, magnified, and inverted image.
Magnification (M): For normal adjustment (final image at infinity), M = fo / fe, where fo is objective focal length and fe is eyepiece focal length.
Magnification: Increases with an increase in the focal length of the objective and a decrease in the focal length of the eyepiece.
Reflecting Telescope (e.g., Newtonian telescope): Uses a concave mirror as the objective to gather light and form an image, often combined with a flat secondary mirror and an eyepiece. Contains only mirrors for the primary light-gathering element.
Resolving Power: The ability of a telescope to form separable images of close objects.
Other Optical Instruments/Concepts:
Dentist's Mirror: Uses a concave mirror to produce a magnified, erect image of teeth.
Parallax Elimination: Achieved by carefully focusing both the eyepiece and the objective of an instrument.
Photoelectric Effect / Photovoltaic Cells: Convert light energy directly into electrical energy. Used in solar cells and photocells (e.g., in fire alarms).
CT Scan (Computed Tomography): Uses Xrays to create detailed cross-sectional images of the body.
Electromagnetic Spectrum: Gamma rays have shorter wavelengths than X-rays.

Polarization of light: Brewster's Law, double refraction, optical activity

Polarization of Light: The phenomenon where vibrations of light waves are restricted to a single plane.
Transverse Wave Nature: Polarization unequivocally proves that light waves are transverse, not longitudinal.
Methods of Polarization: Occurs through reflection, refraction (double refraction), scattering, and selective absorption.
Brewster's Law (Polarization by Reflection):

When unpolarized light is incident at a specific angle (Brewster's angle, `ip`) on a transparent surface, the reflected light is completely plane-polarized perpendicular to the plane of incidence.
The reflected ray and the refracted ray are perpendicular to each other at Brewster's angle (`ip + r = 90°`).
Brewster's Angle Formula: `tan(ip) = μ` (refractive index of the medium).

Double Refraction (Birefringence):

Occurs in certain anisotropic crystals (e.g., calcite, quartz) where incident unpolarized light splits into two refracted rays: an Ordinary ray (O-ray) and an Extraordinary ray (E-ray).
Both O-ray and E-ray are plane-polarized, with their planes of vibration perpendicular to each other.
O-ray obeys Snell's Law and travels with constant speed; E-ray generally does not obey Snell's Law and travels with varying speed.

Optical Activity: The property of certain substances (e.g., sugar solution, quartz, tartaric acid) to rotate the plane of polarization of plane-polarized light passing through them. Such substances are called optically active.
Polaroid Applications:

Sunglasses: Reduce glare from reflected sunlight by blocking horizontally polarized light.
3D Glasses: Used for viewing three-dimensional pictures/films to separate images for each eye.

Light's Nature:

Wave Nature: Supported by phenomena like Interference, Diffraction, and Polarization.
Quantum Nature (Particle Nature): Supported by phenomena like Photoelectric effect and Compton effect. Polarization does not support the quantum nature of light.

Primary Colors (Additive): Red (R), Green (G), Blue (B).
Additive Color Mixing: Red + Green + Blue = White light.
Blue Sky: Appears blue due to the scattering of sunlight by atmospheric particles (Rayleigh scattering), which scatters shorter blue wavelengths more effectively.
Infrared (IR) Rays: Useful in long-distance photography, especially through haze or fog, due to their longer wavelength and consequently less scattering.

Prisms and Dispersion

Dispersion is the phenomenon where white 1. light splits into its constituent colors when passing through a medium, typically a prism. Dispersion occurs because the refractive 2. index of a medium varies with the wavelength (color) of light; different colors travel at different speeds and refract by different amounts. In a prism, violet light deviates the most, 3. while red light deviates the least. The refractive index of glass is highest for 4. violet light and lowest for red light. The speed of light in a prism is slowest for 5. violet light and fastest for red light. White light comprises seven main colors: 6. Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR). The visible light spectrum generally ranges 7. from approximately 380 nanometers (violet) to 750 nanometers (red). Violet light has the shortest wavelength and 8. highest frequency; red light has the longest wavelength and lowest frequency among visible colors. The color of light is determined by its 9. wavelength or frequency. The three primary colors of light are Red, 10. Green, and Blue (RGB). Green is typically found in the middle of the 11. rainbow spectrum. Rainbows are formed by a combination of 12. dispersion, refraction, and internal reflection of sunlight by water droplets. A primary rainbow results from one internal 13. reflection and two refractions within water droplets, appearing brighter with violet on the inner arc and red on the outer. A secondary rainbow results from two intern14. al reflections and two refractions within water droplets, appearing fainter, outside the primary rainbow, and with an inverted color order (red inner, violet outer). Scattering of light is the redirection of light 15. by particles in a medium. Rayleigh scattering states that the intensity of 16. scattered light is inversely proportional to the fourth power of its wavelength (I ∝ 1/λ⁴), meaning shorter wavelengths (blue/violet) scatter more. The blue color of the sky is due to the prefer17. ential scattering of shorter wavelengths (blue and violet) by atmospheric molecules. At very high altitudes, the sky appears dark 18. because there are fewer atmospheric molecules to scatter light. The reddish appearance of the Sun during 19. sunrise and sunset is due to light traveling a longer path through the atmosphere, scattering away most of the shorter (blue) wavelengths, allowing predominantly longer (red) wavelengths to reach the observer. Red light is used in danger signals because it 20. scatters the least by atmospheric particles, enabling it to travel the farthest distance and be visible from a greater range. The Tyndall effect is the scattering of a light 21. beam by colloidal particles. The blue color of sea water is primarily due 22. to the scattering of blue light by water molecules. Twinkling of stars is caused by atmospheric 23. refraction, specifically by fluctuations in the refractive index of Earth's atmosphere due to varying air density and temperature. Optical illusions related to atmospheric 24. phenomena include the apparent size/color of the Sun at dawn/dusk and the twinkling of stars. Common glass significantly absorbs ultravi25. olet (UV) radiation. X-rays are electromagnetic waves and are 26. uncharged.

Reflection and Refraction at plane and spherical surfaces

Light travels rectilinearly in homogeneous and transparent media.
Speed of light (c) is maximum in vacuum ($ \approx 3 \times 10^8 $ m/s), decreases in denser media.
Speed of light in air is largely independent of pressure, temperature, and density.
Laws of Reflection:

Angle of incidence (i) equals angle of reflection (r).
Incident ray, reflected ray, and normal at the point of incidence all lie in the same plane.
These laws apply to all types of reflecting surfaces (plane, spherical, irregular/diffused).

If a mirror is rotated by an angle $ \theta $, the reflected ray rotates by $ 2\theta $.
Deviation of a reflected ray from its original path is $ 180^\circ - 2i $, where $ i $ is the angle of incidence.
Plane Mirrors:

Image formed is virtual, erect, laterally inverted, and of the same size as the object.
Image is formed as far behind the mirror as the object is in front.
Minimum height of a plane mirror to see one's full image is half of the person's height.
Number of images (N) formed by two plane mirrors inclined at an angle $ \theta $: $ N = (360/\theta) - 1 $.

If $ \theta = 90^\circ $ (perpendicular), N = 3.
If $ \theta = 0^\circ $ (parallel), N = infinity.

A periscope uses two parallel plane mirrors.
Focal length of a plane mirror is considered infinite.

Spherical Mirrors:

Relation between Radius of Curvature (R) and Focal Length (f): $ R = 2f $.
Concave Mirror (Converging Mirror):

Focal length is negative (by New Cartesian Sign Convention).
Image properties depend on object position:
Object at infinity: Image at F (real, inverted, highly diminished).
Object at Center of Curvature (C): Image at C (real, inverted, same size).
Object between F and C: Image beyond C (real, inverted, magnified).
Object between Pole (P) and F: Image behind mirror (virtual, erect, magnified).
Uses: Headlights (to send parallel rays), searchlights, dentist mirrors, shaving mirrors (for magnified images).

Convex Mirror (Diverging Mirror):

Focal length is positive (by New Cartesian Sign Convention).
Always forms a virtual, erect, diminished image, located between P and F.
Uses: Rear-view mirrors in vehicles (provides a wider field of view).

Magnification (m) for spherical mirrors: $ m = -v/u = hi/ho $, where $ v $ is image distance, $ u $ is object distance, $ hi $ is image height, $ ho $ is object height.
Parabolic concave mirrors are used in headlights/searchlights to produce a perfectly parallel beam of light.

Refraction of Light:

Bending of light as it passes from one transparent medium to another due to a change in its speed.
Laws of Refraction (Snell's Law):

Incident ray, refracted ray, and normal at the interface all lie in the same plane.
Ratio of sine of angle of incidence (sin i) to sine of angle of refraction (sin r) is a constant for a given pair of media ($ n1 \sin i = n2 \sin r $).

When light travels from a rarer medium to a denser medium:

Its speed decreases.
It bends towards the normal.
Its wavelength decreases.

When light travels from a denser medium to a rarer medium:

Its speed increases.
It bends away from the normal.
Its wavelength increases.

The frequency of light does not change during refraction.
If light is incident normally (angle of incidence = 0°), it passes undeviated (angle of refraction = 0°).

Refractive Index (n):

Measures the optical density of a medium. $ n = c/v $, where $ c $ is the speed of light in vacuum and $ v $ is the speed of light in the medium.
Relative refractive index of medium 2 with respect to medium 1 ($ n{21} $) = $ n2/n1 = v1/v2 $.
Absolute refractive index of a medium (w.r.t. vacuum) is always greater than 1.
Typical values: Air $ \approx 1.0003 $, Water $ \approx 1.33 $, Glass $ \approx 1.5 $.
Higher refractive index means light travels slower in that medium.

Phenomena due to Refraction:

Twinkling of stars: Caused by atmospheric refraction due to continuous changes in the refractive index of Earth's atmosphere.
Apparent depth: Objects submerged in water appear shallower (closer to the surface) due to refraction when viewed from air. A stick partially dipped in water appears bent.
Advanced sunrise and delayed sunset: Sun is visible approximately 2 minutes before actual sunrise and 2 minutes after actual sunset due to atmospheric refraction.
Mirage: An optical illusion caused by atmospheric refraction and total internal reflection, typically in hot regions.

Scattering of Light:

Blue sky: Due to the scattering of shorter wavelengths (blue light) more effectively by atmospheric particles.
Reddish appearance of Sun at sunrise/ sunset: When the Sun is near the horizon, light travels a longer path through the atmosphere, scattering away most of the blue light, leaving predominantly red light to reach our eyes.
Red light as danger signal: Red light scatters the least among visible colors, allowing it to travel further through fog, smoke, etc., and remain visible.

Colors of Objects:

Opaque objects appear to be a certain color because they reflect that color and absorb all other colors.
A black object absorbs all colors and reflects none.
Primary colors of light are Red, Green, and Blue (RGB). Combining Red and Green light yields Yellow light.

Luminous and Non-Luminous Objects:

Luminous objects emit their own light (e.g., Sun, electric lamp, candle flame).
Non-luminous objects reflect light from other sources (e.g., Moon, planets, Jupiter).

Thin and Thick Lenses, lens combinations

Lenses - Types & Properties:

Convex Lens (Converging Lens): Thicker in middle, thinner at edges. Converges parallel rays. Has positive focal length (+f) and positive power (+P).

Image Formation (Convex Lens):
Object between F and Optical Centre (O): Virtual, erect, magnified image (used as magnifying glass).
Object at F: Image at infinity.
Object between F and 2F: Real, inverted, magnified image, beyond 2F.
Object at 2F: Real, inverted, same-size image, at 2F. Minimum object-image distance is 4f.
Object beyond 2F: Real, inverted, diminished image, between F and 2F.
Used to correct Hypermetropia (farsightedness).
Can produce both real and virtual images; images can be equal, greater, or smaller than object size.

Concave Lens (Diverging Lens): Thinner in middle, thicker at edges. Diverges parallel rays. Has negative focal length (-f) and negative power (P).

Image Formation (Concave Lens): Always forms virtual, erect, diminished images, on the same side as the object.
Used to correct Myopia (nearsightedness).

Air bubble in water: Acts as a concave (diverging) lens due to refractive index difference (nwater > nair).
Materials for Lenses: Must be transparent (e.g., glass, water, plastic, sapphire). Opaque materials (e.g., aluminium, clay) cannot be used.
Thick glass plate: Behaves like a plane mirror for basic optical interactions.

Lens Terminology & Formulas:

Focal Length (f): Distance from the optical centre to the principal focus. Sign convention: +f for convex, -f for concave.
Power of a Lens (P): Measured in Dioptre (D). P = 1/f (where f is in meters) or P = 100/f (where f is in cm).

Positive Power (+P) indicates a convex (converging) lens.
Negative Power (-P) indicates a concave (diverging) lens.
Larger absolute power (|P|) means stronger bending of light rays and smaller focal length.
Sun glasses have 0 Dioptre power.

Lens Formula (Thin Lens): 1/f = 1/v 1/u, where u = object distance, v = image distance, f = focal length. (New Cartesian Sign Convention applies).
Magnification (m): m = v/u = hi / ho (image height / object height).

+m: Image is virtual and erect.
-m: Image is real and inverted.
|m| > 1: Magnified image.
|m| < 1: Diminished image.
|m| = 1: Same size image.

Lens Maker's Formula: 1/f = (n-1) (1/ R1 - 1/R2), where n is refractive index, R1 and R2 are radii of curvature of the lens surfaces.
Aperture: The effective diameter of the circular outline of a spherical lens. It is independent of its radius of curvature or focal length.

Lens Combinations (Thin Lenses in Contact):

Equivalent Power (Peq): Peq = P1 + P2 + P3 + ...
Equivalent Focal Length (Feq): 1/Feq = 1/f1 + 1/f2 + 1/f3 + ...
For two thin lenses separated by distance 'd': 1/Feq = 1/f1 + 1/f2 - d/ (f1f2).

Human Eye Defects & Correction:

Myopia (Nearsightedness): Distant vision is blurred. Corrected using a concave lens.
Hypermetropia (Farsightedness): Near vision is blurred. Corrected using a convex lens.
Presbyopia: Age-related difficulty focusing on near objects. Corrected using bifocal lenses.
Astigmatism: Blurred vision at all distances due to uneven corneal curvature. Corrected using cylindrical lenses.
Cataract: Clouding of the eye's natural lens. Corrected by surgery.

Applications:

Magnifying glass: Uses a convex lens.
Camera: Main optical component is a lens.

Total Internal Reflection and applications

Total Internal Reflection (TIR) occurs when 1. light travels from a denser medium to a rarer medium. For TIR to occur, the angle of incidence (i) in 2. the denser medium must be greater than the critical angle (θc) for the pair of media. In an ideal TIR, there is no loss of intensity of 3. light. Critical Angle (θc) is the angle of incidence 4. in the denser medium for which the angle of refraction in the rarer medium is 90 degrees. Applications of TIR: 5. Brilliance of Diamond: Due to its very high 6. refractive index (approx. 2.42) and a correspondingly low critical angle (approx. 24.4 degrees), light entering a diamond undergoes multiple total internal reflections, making it sparkle. (Note: Diamond has a very high, not low, refractive index). Optical Fibres: Work on the principle of 7. multiple total internal reflections. Light signals are transmitted along the fibre core (denser medium) by repeatedly reflecting off the cladding (rarer medium). Endoscopy: Medical instruments (endo8. scopes) use optical fibres to examine internal organs, relying on total internal reflection for image transmission. Mirage: An optical illusion observed primar9. ily in hot conditions (e.g., deserts, hot roads). It is caused by both refraction and total internal reflection of light due to varying densities (and thus refractive indices) of air layers near the ground. Rainbow Formation: Involves refraction, dis10. persion, and total internal reflection of sunlight within water droplets. Totally Reflecting Prisms: Used in optical 11. instruments like binoculars and periscopes to reflect light without absorption, based on TIR. Reflectors on Traffic Signals/Glass Beads: 12. Utilize TIR to reflect light efficiently. TIR can take place when light travels from a 13. medium with a higher refractive index to a medium with a lower refractive index (e.g., diamond to glass, as ndiamond > nglass). It cannot occur when light travels from rarer to denser medium (e.g., air to water, air to glass, water to glass).

Wave Nature of Light: Huygens' Principle

Electromagnetic Spectrum (EMS) components (decreasing wavelength, increasing frequency/energy): Gamma rays > X-rays > Ultraviolet (UV) > Visible Light > Infrared (IR) > Microwaves > Radio waves.
All EM waves travel at the speed of light 'c' in vacuum and are transverse in nature.
EM waves are not deflected by electric or magnetic fields.
Visible Spectrum: Wavelength range is approximately 3900 Å - 7600 Å (or 400 nm 700 nm).
Within the visible spectrum, red light has the longest wavelength and lowest frequency/ energy; violet light has the shortest wavelength and highest frequency/energy (VIBGYOR).
When light travels from one medium to another, its frequency remains constant, but its velocity and wavelength change.
Light exhibits wave-particle duality.
Wave Nature is explained by phenomena like Diffraction, Interference, and Polarization. Huygens' principle is foundational to wave theory of light.
Particle Nature is explained by phenomena like Photoelectric effect, Compton effect, and Black-body radiation. Light exists as tiny energy packets called photons.
Photon energy (E) is directly proportional to its frequency (f) and inversely proportional to its wavelength (λ): E = hf = hc/λ (where h is Planck's constant, c is speed of light in vacuum).
Higher frequency (shorter wavelength) corresponds to higher energy per photon.
X-rays: Have very short wavelengths (order of 1 Å or 0.1 nm), high frequency, and high energy. They possess high penetration length in matter.
Ultraviolet (UV) radiation: Has shorter wavelength and higher energy than visible light.
Infrared (IR) radiation: Has longer wavelength and lower energy than visible light. Sun's radiation includes light and short-wavelength infrared.
Photoelectric Effect: Emission of electrons from a metallic surface when illuminated by light. Occurs when incident photon energy (hf) is greater than or equal to the metal's work function (W). For zero-velocity photoelectrons, hf = W.
Given work function W = 4 eV, the threshold wavelength (λ) is hc/W. Using hc ≈ 12400 eV Å, λ = 12400 eV Å / 4 eV = 3100 Å.
Raman Effect: An inelastic scattering phenomenon where photons interact with molecules, leading to a change in the frequency (and thus energy) of the scattered light. Occurs in pure liquids, gases, and solids.
Scattering of Light:

Tyndall Effect: Scattering of light by colloidal particles, making the path of the light beam visible.
Atmospheric diffusion/scattering of light (e.g., making sunbeams visible) is primarily due to dust particles.
Rayleigh Scattering: Explains blue sky; scattering by particles much smaller than the wavelength of light, preferentially scattering shorter wavelengths (blue).

Fluorescence: A process where a substance absorbs higher-energy radiation (e.g., UV light) and re-emits it as lower-energy visible light.
The Balmer series in the emission spectrum of hydrogen belongs to the visible region of the electromagnetic spectrum.

7. Heat and Thermodynamics

Practice MCQs: Optics

Diffraction: Fraunhofer and Fresnel diffraction, diffraction grating

Soap solution used for cleaning purpose appears cloudy. This is due to the fact that soap micelles can
Asked in: UPSC CDS 2 2019 · Horticulture Development Officer(HDO) 2018
1. refract light
2. scatter light
3. diffract light
4. polarize light
Show answer
Answer: B. scatter light
B. Gravitational force C. Surface tension D. Viscosity Ans: C Surface tension is the property of the surface of a liquid that allows it to resist an external force due to the cohesive nature of its molecules. In a ball pen, surface tension helps to keep the ink in place within the reservoir and ensures a controlled flow of ink onto the ball and then to the writing surface.
Ball pen functions on the principle of
Asked in: TOURIST INFORMATION OFFICER(TIO) 2017
1. Boyle's Law
2. Gravitational force
3. Surface tension
4. Viscosity
Show answer
Answer: C. Surface tension
Ball pens utilize the principle of surface tension for their functioning. The cloudy appearance of soap solution is due to the scattering of light by soap micelles (Tyndall effect).
Which one of the following combinations of source and screen would produce sharpest shadow of an opaque object?
Asked in: UPSC CDS 2 2020
1. A point source and an opaque screen
2. An extended source and an opaque screen
3. A point source and a transparent screen
4. An extended source and a transparent screen
Show answer
Answer: A. A point source and an opaque screen
A point source of light creates a sharp shadow (umbra) without a penumbra, unlike an extended source which creates both umbra and penumbra.
Diffusion of light in the atmosphere takes place due to :
Asked in: HPPSC HPAS Prelims 2013 · UPPCS Prelims 2003
1. Carbon dioxide
2. Dust particles
3. Helium
4. Water vapours
Show answer
Answer: B. Dust particles
Atmospheric scattering of light occurs when light rays interact with particles or molecules in the atmosphere. This scattering redirects light in different directions, causing the diffusion of light. Different types of scattering occur depending on the size of the scattering particles relative to the wavelength of light. Rayleigh scattering occurs by gas molecules (much smaller than wavelength) and is responsible for the blue colour of the sky. Mie scattering occurs by larger particles like dust, smoke, and water droplets (comparable in size to wavelength) and is less wavelength-dependent, contributing to phenomena like haze and the white appearance of clouds. Dust particles are significant contributors to Mie scattering.
वाुमण्डल में प्रकाश का ͪवसिरण ͩकसि कारण होता है ?
Asked in: HPPSC HPAS Prelims 2013
1. काबर्यन डाइआँक्सिाइड
2. धूलि के कण
3. हीlलियम
4. जलिवाष्प
Show answer
Answer: B. धूलि के कण
Atmospheric scattering is caused by gases and particles. Dust particles are significant scatterers of light, contributing to phenomena like haze and varying sky colours at different times of day.
When a CD (compact disc used in audio and video systems) is seen in sunlight, rainbow like colours are seen. This can be explained on the basis of the phenomenon of :
Asked in: HPPSC HPAS Prelims 2014 · UPPCS Prelims 2000
1. Reflection and diffraction
2. Reflection and transmission
3. Diffraction and transmission
4. Refraction, diffraction and transmission
Show answer
Answer: A. Reflection and diffraction
The rainbow-like colours seen when a CD is viewed in sunlight are caused by the reflection of light off the finely spaced tracks of pits and lands on the disc surface. These tracks act as a diffraction grating. When white light is reflected and passes over these tiny structures, it is diffracted, meaning it bends and splits into its constituent wavelengths (colours) at slightly different angles, creating the spectrum effect.
Which one of the following wavelengths corresponds to the wavelength of X-rays?
Asked in: UPSC NDA 2 2022
1. 500 nm
2. 5000 nm
3. 100 nm
4. 1 nm
Show answer
Answer: D. 1 nm
Comparing the given options to the typical range of X-ray wavelengths (0.01 nm - 10 nm): A) 500 nm falls in the visible light spectrum. B) 5000 nm (5 µm) falls in the infrared spectrum. C) 100 nm falls in the ultraviolet spectrum. D) 1 nm falls within the typical range of X-ray wavelengths.
Tyndall effect appears due to which one of the following properties of light?
Asked in: UPSC CISF AC EXE 2020
1. Reflection of light
2. Diffraction of light
3. Polarization of light
4. Scattering of light
Show answer
Answer: D. Scattering of light
Tyndall effect is the scattering of light by particles in a colloid or a very fine suspension.

Fiber Optics: Principles and applications

Consider the following statements : (i) Optical fibre communication is based on the principle of refraction of light. (ii) Optical fibre communication is based on the principle of total internal reflection of light. (iii) In optical fibre communication, light is the carrier wave. (iv) Optical fibre permits communication over short distances only. Select the correct statement(s) using the codes given below.
Asked in: APSC CCE Prelims Exam 2022
1. Only (i) and (iv)
2. Only (ii), (iii) and (iv)
3. Only (ii) and (iii)
4. Only (iii) and (iv)
Show answer
Answer: C. Only (ii) and (iii)
Optical fibre communication relies on Total Internal Reflection for transmitting light signals and is known for its ability to carry data over long distances at high speeds.
A photoelectric cell is a device which
Asked in: BPSC Prelims Exam 69th
1. converts light energy into electric energy
2. converts electric energy into light energy
3. stores light energy
4. None of the above
Show answer
Answer: A. converts light energy into electric energy
When light falls on a suitable material in the photoelectric cell, it causes electrons to be emitted, generating an electric current.
Light rays move in straight lines. But through an optical fibre, they can move in any type of zigzag path because
Asked in: UPSC NDA 2 2019
1. the holes through the fibre are extremely fine.
2. light rays are absorbed at the entry end and relieved at the exit end of the fibre.
3. scattering of light occurs inside the fibre.
4. successive total internal reflections occur as a ray moves through the fibre.
Show answer
Answer: D. successive total internal reflections occur as a ray moves through the fibre.
Optical fibres work on the principle of Total Internal Reflection (TIR). The fibre consists of a core with a higher refractive index surrounded by cladding with a lower refractive index. Light entering the core at suitable angles strikes the core- cladding interface at angles greater than the critical angle, causing the light to be reflected back into the core. This process repeats along the length of the fibre, guiding the light even if the fibre is bent.
Which one of the following phenomenon is used in optical fibres?
Asked in: UPPSC Upper Mains 2015
1. Interference
2. Refaction
3. Total internal reflection
4. Polaristion
Show answer
Answer: C. Total internal reflection
Optical fibres work on the principle of total internal reflection. Light signals are transmitted through the fibre by continuously reflecting off the inner walls of the fibre, experiencing total internal reflection at each bounce.
Endoscopy, a technique used to explore the stomach or other inner parts of the body is based on the phenomenon of
Asked in: UPPCS Prelims 1999
1. total internal reflection
2. interference
3. diffraction
4. polarisation
Show answer
Answer: A. total internal reflection
Modern flexible endoscopes use optical fibers to transmit light into the body cavity and transmit the image back out. Light travels along these fibers by repeatedly undergoing total internal reflection at the interface between the fiber core and its cladding. When light strikes the interface at an angle greater than the critical angle, it is reflected back into the core, allowing it to be guided along the curved path of the fiber.
Which one of the following does a TV remote control unit use to operate a TV set?
Asked in: UPPCS Prelims 2000
1. Light waves
2. Sound waves
3. Micro waves
4. Radio waves
Show answer
Answer: A. Light waves
- TV remotes typically use infrared (IR) signals. - Infrared radiation is a type of electromagnetic wave. - Infrared is often considered part of the 'light' spectrum, distinct from radio or microwaves.
In fiber optics communication the signal travels in the form of:
Asked in: RPSC RAS Prelims 2012
1. Light wave
2. Radiowave
3. Mircrowave
4. Electric Wave
Show answer
Answer: A. Light wave
Fiber optic cables transmit data by sending pulses of light through thin strands of glass or plastic fiber. The electrical signals are converted into light signals (pulses) at the transmitting end and converted back into electrical signals at the receiving end.
In One of the followings infra red light is not used. This is
Asked in: law officier 2025
1. Breath analyzer
2. In the detections of forged documents
3. Night vision apparatus
4. In green house to keep plants warm
Show answer
Answer: A. Breath analyzer
Infrared spectroscopy is a technique used to identify compounds by analyzing the absorption of infrared radiation. It is effectively used in detecting forged documents (B) by examining ink or paper composition, and in night vision apparatus (C) to detect heat signatures, and in greenhouses (D) to trap heat and keep plants warm.

Holography

Which of the following is used to take 3- dimen-sional pictures?
Asked in: UPPSC Upper Mains 2008
1. Photography
2. Holography
3. Radiography
4. None of the above
Show answer
Answer: B. Holography
Holography is a technique that allows the light scattered from an object to be recorded and later reconstructed so that it appears as if the object is in the same relative position as when it was recorded, thus producing a three-dimensional image called a hologram. Photography, in contrast, typically captures a two-dimensional image. Radiography uses X-rays to create internal images, usually also presented as 2D projections.
Method for recording three dimension photograph using laser beam is:
Asked in: RPSC RAS Prelims 1996
1. Halography
2. Xerography
3. Videography
4. Audiography
Show answer
Answer: A. Halography
Holography is a photographic technique that records the light scattered from an object, and then presents it in a way that appears three-dimensional when viewed. It involves the use of a laser beam to create an interference pattern that is recorded on a medium (the hologram).

Interference: Young's double-slit experiment, thin films, Newton's Rings

When a soap film on the water is seen in the daytime, it shows beautiful colours. This phenomenon is due to
Asked in: BPSC Prelims Exam 67th Cancelled
1. diffraction of light
2. refraction of light
3. polarization of light
4. interference of light
Show answer
Answer: D. interference of light
When white light strikes a thin film like a soap film, light is reflected from both the top and bottom surfaces of the film. These reflected light waves interfere with each other. Depending on the thickness of the film, the wavelength of light, and the angle of incidence, certain wavelengths will interfere constructively (enhancing the color) and others destructively (canceling the color). Since white light is composed of many wavelengths (colors), the film appears colored as different colors interfere constructively at different points due to variations in the film thickness.
On a rainy day, small oil films on water show brilliant colours. This is due to
Asked in: GPSC Prelims CSP 2 2023
1. dispersion
2. interference
3. diffraction
4. polarization
Show answer
Answer: B. interference
Thin-film interference occurs when light waves reflect off the top and bottom surfaces of a thin layer (like the oil film), and these reflected waves interfere with each other, producing constructive or destructive interference depending on the wavelength of light, the thickness of the film, and the angle of incidence.
In Fresnel's Biprism, the value of wavelength (λ) depends upon
Asked in: MPSC CS Prelims 2018
1. The distance between two coherent sources
2. The fringe-width
3. The distance between slit and eyepiece of tele‐ scope
4. All of the above
Show answer
Answer: D. All of the above
- Fresnel's Biprism produces two virtual coherent sources from a single source by refracting light through two prisms placed base-to-base. - These two coherent sources interfere, producing interference fringes in the region where the light beams overlap. - The fringe width (β), which is the distance between two consecutive bright or dark fringes, is given by the formula: β = λD / d. - In the experiment, β, D, and d are measured. - β is measured using a microscope by finding the positions of several fringes. - D is the distance from the slit (source) to the eyepiece (where fringes are observed). - d is the distance between the two virtual coherent sources. This is usually determined by measuring the distance between the images of the slit formed by a convex lens placed between the biprism and the eyepiece (using the displacement method) and the distance D. - To find the wavelength (λ), the formula is rearranged as λ = βd / D. - Therefore, the calculated value of λ depends on the values of β, d, and D.
Two parallel slits 1 millimetre apart are placed 1 metre away from the screen. What is the fringe separation due to interference effect, when a beam of wavelength 500 nanometres is incident on them ?
Asked in: MPSC CS Prelims 2018
1. 5.0 mm
2. 0.5 mm
3. 5.0 cm
4. 0.05 mm
Show answer
Answer: B. 0.5 mm
Fringe separation in double-slit interference is directly proportional to the wavelength and the screen distance, and inversely proportional to the slit separation.
Two slits are made one millimetre apart and the screen is placed one metre away. What is the fringe separation when blue-green light of wavelength 500 nm is used ?
Asked in: UKPSC CS Prelims 2024
1. 0.5 m
2. 0.5 mm
3. 50 mm
4. 50 m
Show answer
Answer: B. 0.5 mm
The fringe separation ($\Delta y$), also known as fringe width, in Young's double-slit experiment is given by the formula $\Delta y = \frac{\lambda D} {d}$, where $\lambda$ is the wavelength of light, $D$ is the distance between the slits and the screen, and $d$ is the distance between the two slits. Given $\lambda = 500$ nm = $500 \times 10^{-9}$ m = $5 \times 10^{-7}$ m, $D = 1$ m, and $d = 1$ mm = $1 \times 10^{-3}$ m. Plugging in the values: $ \Delta y = \frac{(5 \times 10^{-7} \text{ m}) \times (1 \text{ m})}{1 \times 10^{-3} \text{ m}} = \frac{5 \times 10^{-7}}{10^{-3}} \text{ m} = 5 \times 10^{-4} \text{ m}$. Converting to millimeters: $5 \times 10^{-4} \text{ m} \times \frac{1000 \text{ mm}}{1 \text{ m}} = 5 \times 10^{-1} \text{ mm} = 0.5 \text{ mm}$.
The formation of colours in soap bubbles is due to the phenomenon of
Asked in: UPSC CAPF 2013
1. dispersion of light
2. interference of light
3. diffraction of light
4. polarization of light
Show answer
Answer: B. interference of light
The iridescent colours observed in soap bubbles are a result of the interference of light waves. When light strikes the thin soap film, it reflects off both the outer and inner surfaces. These two reflected waves interfere with each other. Due to the slight difference in the path length traveled by the waves (determined by the thickness of the film and the angle of incidence) and the phase change upon reflection, certain wavelengths of light are constructively interfered (enhanced), while others are destructively interfered (cancelled). This results in the appearance of vibrant colours, which vary depending on the local thickness of the soap film.
When beam of Red, blue and green light falls on the sam spot, the color of light becomes:
Asked in: RPSC RAS Prelims 2003
1. Violet
2. Red
3. Yellow
4. White
Show answer
Answer: D. White
Additive colour mixing is the process of creating different colours by mixing lights of different colours, commonly used in displays like televisions and computer monitors.
When an oil film on water is seen in daytime, it shows beautiful colours. This phenomenon is due to
Asked in: Assistant Engineer (Civil) 2018 19 2018
1. diffraction
2. refraction
3. polarization
4. interference
Show answer
Answer: D. interference
Thin-film interference occurs when light waves reflecting from the top and bottom surfaces of a thin film interfere. This phenomenon is responsible for the iridescent colours seen in soap bubbles, oil slicks, and the wings of some insects.

Laser: Principles and applications

Which of the following properties is not related to LASER ?
Asked in: HPPSC HPAS Prelims 2010
1. High intensity
2. High coherence
3. High monochromaticity
4. High optical activity
Show answer
Answer: D. High optical activity
LASER (Light Amplification by Stimulated Emission of Radiation) produces light with specific characteristics: High Intensity (concentrated power), High Coherence (photons are in phase), and High Monochromaticity (very narrow range of wavelengths/color). Optical activity is a property of certain materials that can rotate the plane of polarization of light passing through them, not a fundamental property of the light source itself.
Consider the following statements regarding LASER : (a) Laser beam is monochromatic. (b) Laser beam is coherent. (c) Laser beam is highly divergent. (d) In the emission of laser beam stimulated emission is absent. Choose the correct answer from the options given below :
Asked in: KPSC KAS Prelims 2024
1. All of the above
2. (a), (b) and (c) only
3. (b), (c) and (d) only
4. (c) and (d) only
Show answer
Answer: B. (a), (b) and (c) only
Key properties of laser light are monochromaticity, coherence, and low divergence. The emission process relies on stimulated emission.
The full form of LED is
Asked in: UPSC NDA 2 2018
1. Light Emitting Diode
2. Light Emitting Device
3. Light Enhancing Device
4. Light Enhancing Diode
Show answer
Answer: A. Light Emitting Diode
This is a straightforward question testing basic knowledge of common electronic component abbreviations.
In which one of the following devices, the light energy is converted into the electrical energy?
Asked in: UPSC NDA 2 2022
1. Light-emitting diode
2. Laser diode
3. Solar cell
4. Transistor
Show answer
Answer: C. Solar cell
The question asks for a device where light energy is converted into electrical energy. This process is known as the photovoltaic effect. - Light-emitting diode (LED) converts electrical energy into light energy. - Laser diode also converts electrical energy into coherent light energy. - Solar cell (photovoltaic cell) is specifically designed to absorb photons from sunlight and convert their energy into an electric current. - Transistor is a semiconductor device used as an electronic switch or amplifier; it controls the flow of electrical current but does not primarily convert light energy to electrical energy (though some transistors can be light-sensitive, their main function isn't energy conversion). Therefore, the solar cell is the device that converts light energy into electrical energy.
What was the active medium used in the first working laser ever constructed ?
Asked in: OPSC CS Prelims 2021
1. Diamond block
2. Helium-neon gas
3. Ruby rod
4. Carbon dioxide gas
Show answer
Answer: C. Ruby rod
Maiman's laser used a ruby crystal doped with chromium ions. When exposed to intense light from a flash lamp, the chromium ions were excited to higher energy levels. They then emitted photons, which stimulated the emission of more photons, leading to the amplification of light and the production of a coherent laser beam.
The term LASER stands for
Asked in: UPSC CAPF 2018
1. Light Amplification by Stimulated Emission of Radiation
2. Light Amplification by Spontaneous Emission of Radiation
3. Light Amplification by Stimulated Emission of Rays
4. Light Amplification by Stimulated Energy of Radiation
Show answer
Answer: A. Light Amplification by Stimulated Emission of Radiation
LASER is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The full form reflects the fundamental principle behind its operation.
Which one of the following statements on photoelectric effect is NOT correct ?
Asked in: UPSC CAPF 2020
1. Albert Einstein received the Nobel Prize in Physics for explaining photoelectric effect
2. For each metal, there is a threshold frequency v₀ below which this effect is not observed
3. At a frequency v > v₀, the kinetic energy of ejected electrons does not change on increasing the frequency of incident light
4. The number of electrons ejected is proportion‐ al to the intensity of incident light
Show answer
Answer: C. At a frequency v > v₀, the kinetic energy of ejected electrons does not change on increasing the frequency of incident light
According to Einstein's explanation of the photoelectric effect, the maximum kinetic energy (KE_max) of the ejected electrons is given by KE_max = h(v - v₀), where h is Planck's constant, v is the frequency of incident light, and v₀ is the threshold frequency. This equation shows that for v > v₀, the maximum kinetic energy is directly proportional to the frequency of the incident light; increasing v leads to increased KE_max.
With reference to street-lighting, how do sodium lamps differ from LED lamps? 1. Sodium lamps produce light in 360 degrees but it is not so in the case of LED lamps. 2. As street-lights, sodium lamps have longer life span than LED lamps. 3. The spectrum of visible light from sodium lamps is almost monochromatic while LED lamps offer significant colour advantages in street-lighting. Select the correct answer using the code given below.
Asked in: UPSC IAS 2021
1. 3 only
2. 2 only
3. 1 and 3 only
4. 1, 2 and 3
Show answer
Answer: C. 1 and 3 only
- Sodium lamps emit light omnidirectionally, while LEDs are directional. - LEDs have a significantly longer lifespan than sodium lamps. - Sodium lamps provide poor color rendering (monochromatic light), while LEDs offer much better color rendering.

Optical Instruments: microscopes, telescopes, human eye

Consider the electromagnetic radiations having wavelengths 200 nm, 500 nm and 1000 nm. Which wavelength(s) of the following can make visual sensation to a human eye?
Asked in: UPSC CDS 1 2017
1. 200 nm and 500 nm
2. 500 nm and 1000 nm
3. 500 nm only
4. 200 nm and 1000 nm
Show answer
Answer: C. 500 nm only
- Wavelengths outside the visible spectrum (like 200 nm which is ultraviolet, and 1000 nm which is infrared) do not cause a visual sensation in humans. - Out of the given wavelengths, only 500 nm falls within the visible spectrum (approximately 400-700 nm), corresponding to green light.
Magnification is
Asked in: UPSC CDS 2 2019
1. actual size of specimen / observed size
2. observed size of specimen / actual size
3. actual size of specimen - observed size
4. actual size of specimen × observed size
Show answer
Answer: B. observed size of specimen / actual size
Magnification = Observed size of specimen / Actual size of specimen.
For normal eyes what is the minimum distance for clear vision ?
Asked in: CGPSC Prelims Exam 2013
1. 10 cm.
2. 15 cm.
3. 20 cm.
4. 25 cm.
Show answer
Answer: D. 25 cm.
For a normal human eye, the minimum distance at which objects can be seen clearly without strain is called the near point or the least distance of distinct vision. This distance is approximately 25 cm for a healthy young adult.
A person is unable to see distant objects clearly: Statement - I: The person is suffering from hypermetropia. Statement - II: This defect may be caused by excessive curvature (curvature) of the eye lens or elongation (elongation) of the eyeball. Statement - III: The defect can be corrected by using a concave (concave) lens of proper power. Statement - IV: The defect can be corrected by using a convex (convex) lens of proper power. Which of the above combinations is correct?
Asked in: HAS Prelims 2021
1. Statement - I and III
2. Statement - II and IV
3. Statement - I and IV
4. Statement - II and III
Show answer
Answer: D. Statement - II and III
The condition where a person is unable to see distant objects clearly is called Myopia (nearsightedness). Statement I: Hypermetropia (farsightedness) is the difficulty in seeing near objects clearly. Therefore, stating the person is suffering from hypermetropia is incorrect for the described symptom. Statement II: Myopia is caused when light is focused in front of the retina. This can happen if the eyeball is too long (axial myopia) or if the cornea or lens is too curved, causing excessive refraction (refractive myopia). The statement correctly describes potential causes of this defect. Statement III: Myopia is corrected by using a concave (diverging) lens. A concave lens diverges light rays before they enter the eye, shifting the focal point backward onto the retina. This statement correctly describes the correction for this defect. Statement IV: A convex (converging) lens is used to correct hypermetropia or presbyopia, where light is focused behind the retina. Using a convex lens would worsen myopia. This statement is incorrect.
A person is not able to see distant objects clearly. Statement - I : The person is suffering from Hypermetropia. Statement - II : The defect may have been caused due to excessive curvature of the eye lens or elongation of the eye ball. Statement - III : The defect can be corrected by using a concave lens of suitable power. Statement - IV : The defect can be corrected by using a convex lens of suitable power. Which of the following combinations of above statements is correct?
Asked in: HAS Prelims 2021
1. Statements - I and II
2. Statements - II and IV
3. Statements - I and III
4. Statement - II and III
Show answer
Answer: D. Statement - II and III
- Statement I: The person is suffering from Hypermetropia. Hypermetropia (farsightedness) is difficulty seeing near objects clearly, while distant vision is usually good. Since the person cannot see distant objects clearly, this statement is false. - Statement II: The defect may have been caused due to excessive curvature of the eye lens or elongation of the eye ball. Myopia is caused when the eye converges light too strongly, causing the focal point to fall in front of the retina instead of on it. This can be due to an eyeball that is too long (axial myopia) or a lens/cornea that is too curved (refractive myopia). This statement correctly describes causes of Myopia. This statement is true about the defect described. - Statement III: The defect can be corrected by using a concave lens of suitable power. A concave (diverging) lens is used to correct Myopia. It diverges the incoming light rays before they enter the eye, shifting the focal point back onto the retina. This statement is true about the correction for the defect. - Statement IV: The defect can be corrected by using a convex lens of suitable power. A convex (converging) lens is used to correct Hypermetropia. This statement is false about the correction for Myopia.
The method of CT scan used in modern medical imaging uses
Asked in: MPSC CS Prelims 2012
1. Alpha rays
2. Sound waves
3. X-rays
4. Microwaves
Show answer
Answer: C. X-rays
Computed Tomography (CT) scanning is a medical imaging technique that uses X-rays to create cross- sectional images (slices) of the body.
In an optical instrument like telescope, the combination of field lens and eye lens forms
Asked in: MPSC CS Prelims 2015
1. an objective
2. compound eye-piece
3. magnifier
4. None of the above
Show answer
Answer: B. compound eye-piece
- An optical instrument like a telescope has two main parts: the objective and the eyepiece. - The eyepiece magnifies the image formed by the objective. - An eyepiece consisting of multiple lenses, such as a field lens and an eye lens, is called a compound eye-piece.
What focal length should the spectacles have for a person for whom the least distance of distinct vision is 50 cm ?
Asked in: MPSC CS Prelims 2018
1. 50 cm, concave
2. 50 cm, convex
3. 25 cm, concave
4. 25 cm, convex
Show answer
Answer: B. 50 cm, convex
Hypermetropia is corrected using a convex lens. The focal length is calculated such that an object at the normal near point (25 cm) forms a virtual image at the person's abnormal near point.

Polarization of light: Brewster's Law, double refraction, optical activity

Which of the following are the primary colors ?
Asked in: HPPSC HPAS Prelims 2016
1. Yellow, Green, Blue
2. Red, Green, Blue
3. Red, Green, White
4. Red, Magenta, Blue
Show answer
Answer: B. Red, Green, Blue
Mixing these three primary colors of light in varying proportions can produce a wide range of other colors, including white when mixed equally.
Match List-I with List-II: List—I List—II (a) Polaroid sun glasses de‐ (i) Double recrease glare on a sunny day fraction because they (b) The substances that rotate (ii) Block a the plane of polarization are portion of said to be light (c) Which phenomenon causes (iii) Optically the polarization of light active (d) Quantum nature of light is (iv) Diffracnot supported by the phetion of light nomenon of (v) Interference of light Choose the correct answer from the options given below:
Asked in: KPSC KAS Prelims 2024
1. (a) (ii), (b) (iii), (c) (iv), (d) (v)
2. (a) (iii), (b) (ii), (c) (iv), (d) (v)
3. (a) (i), (b) (v), (c) (iii), (d) (ii)
4. (a) (ii), (b) (iii), (c) (i), (d) (v)
Show answer
Answer: D. (a) (ii), (b) (iii), (c) (i), (d) (v)
Polarization is a property of transverse waves, specifying the geometrical orientation of the oscillations. Optical activity is the property of some materials to rotate the plane of polarized light. Interference and diffraction are wave phenomena.
Which one of the following optical phenomena supports that the light is a transverse wave ?
Asked in: UPSC NDA 2 2023
1. Refraction
2. Diffraction
3. Interference
4. Polarization
Show answer
Answer: D. Polarization
- Polarization is the restriction of wave oscillations to a specific plane. - Only transverse waves can be polarized. - Refraction, diffraction, and interference occur for both wave types.
While watching three-dimensional pictures we need to wear
Asked in: PPSC CS Prelims 2018
1. Magnifying glass
2. Convex glass
3. Polaroid glass
4. None of the above
Show answer
Answer: C. Polaroid glass
- Passive 3D systems project two images simultaneously, polarized differently (e.g., one horizontally, one vertically, or circularly). - Polaroid glasses have lenses with corresponding polarizing filters that allow only one of the projected images to reach each eye. - The brain then combines these slightly different images from each eye to create the perception of depth, resulting in a 3D effect.
Spectacles used for viewing 3-Dimensional films have :
Asked in: UPSC CAPF 2010
1. convex lens.
2. polaroids.
3. concave lens.
4. bifocal lens.
Show answer
Answer: B. polaroids.
- One common method for displaying 3D films uses polarized light. Two images are projected simultaneously, each polarized differently (e.g., linearly polarized at 90 degrees to each other, or circularly polarized in opposite directions). - The 3D glasses contain polarizing filters (polaroids) that match the polarization of the projected images. The filter over the left eye allows only the image intended for the left eye to pass through, and the filter over the right eye allows only the image intended for the right eye. - The brain then combines these two slightly different images to create the perception of depth.
Red + Green + Blue = ?
Asked in: WBPSC CS Prelims 2018
1. White
2. Black
3. Maroon
4. Blue
Show answer
Answer: A. White
This question refers to additive colour mixing, which is the process of mixing colours of light. Red, Green, and Blue (RGB) are the primary colours in additive mixing. When these three primary colours of light are mixed together in equal proportions, the result is white light. This principle is used in devices like televisions, computer monitors, and phone screens.
Polarisation of light proves the
Asked in: WBCS Main Paper 4 2022
1. longitudinal nature of light
2. quantum nature of light
3. corpuscular nature of light
4. transverse nature of light
Show answer
Answer: D. transverse nature of light
Only transverse waves, where oscillations are perpendicular to the direction of propagation, can be polarized. Longitudinal waves, where oscillations are parallel to the direction of propagation (like sound waves), cannot be polarized. Light can be polarized, proving its transverse nature.
Which of the following are primary colours ?
Asked in: Forest Range Officer Exam 2012
1. Blue, Green, Yellow
2. Red, Blue and Green
3. Red, Black and Green
4. Green, White, Black
Show answer
Answer: B. Red, Blue and Green
The concept of primary colors differs depending on whether you are discussing additive color mixing (light) or subtractive color mixing (pigments/ paints). This question refers to additive primaries.

Prisms and Dispersion

The optical phenomenon that is primarily responsible for the observation of rainbow on a rainy day is
Asked in: UPSC CDS 1 2017
1. diffraction
2. interference
3. dispersion
4. reflection
Show answer
Answer: C. dispersion
- When white sunlight enters a water droplet, it is refracted (bent) because the speed of light changes as it passes from air to water. - As sunlight is composed of different colors (wavelengths), and the refractive index of water is slightly different for each color, the white light is split into its constituent colors. This splitting of white light into its spectrum is called dispersion. Dispersion is crucial because it separates the colors we see in a rainbow. - The dispersed light then reflects off the inner back surface of the water droplet. - Finally, the light is refracted again as it exits the droplet into the air, further enhancing the separation of colors.
When the Sun is near the horizon during the morning or evening, it appears reddish. The phenomenon that is responsible for this observation is
Asked in: UPSC CDS 1 2018
1. reflection of light
2. refraction of light
3. dispersion of light
4. scattering of light
Show answer
Answer: D. scattering of light
- Rayleigh scattering is inversely proportional to the fourth power of the wavelength (scattering ∝ 1/λ⁴), meaning shorter wavelengths are scattered much more than longer wavelengths. - The path length of sunlight through the atmosphere is longest at sunrise and sunset.
How many internal reflections of light take place in the formation of primary rainbow ?
Asked in: UPSC CDS 1 2021
1. 0
2. 1
3. 2
4. More than 2
Show answer
Answer: B. 1
The light path for a primary rainbow within a raindrop includes one internal reflection. The sequence is: refraction (entry) → reflection (internal) → refraction (exit). This single internal reflection, combined with refraction and dispersion, separates the light into its constituent colours, forming the rainbow arc.
The phenomenon of rainbow formation by water droplets involves :
Asked in: UPSC CDS 2 2024
1. single refraction of sunlight
2. one internal reflection of sunlight
3. two internal reflections of sunlight
4. both refraction and internal reflection of sun‐ light
Show answer
Answer: D. both refraction and internal reflection of sun‐ light
A rainbow is formed when sunlight interacts with raindrops. Sunlight is first refracted (bent) as it enters the water droplet. It is then internally reflected off the back inner surface of the droplet. Finally, it is refracted again as it exits the droplet. The different wavelengths (colors) of light are refracted at slightly different angles (dispersion), separating the white light into the spectrum of colors observed in a rainbow.
Read the following statements and choose the correct option: (i) For passengers flying at very high altitudes, sky appears dark. (ii) Scattering is not prominent at such heights.
Asked in: CGPSC Prelims Exam 2022
1. (i) and (ii) both are true, and (ii) is correct reasoning of (i)
2. (i) and (ii) both are true, but (ii) is not correct reasoning of (i)
3. (i) is correct and (ii) is incorrect
4. (i) is incorrect and (ii) is correct
Show answer
Answer: A. (i) and (ii) both are true, and (ii) is correct reasoning of (i)
- Statement (i) is true. When flying at very high altitudes (like in a jet plane or from space), the sky above appears dark, almost black, rather than blue. - Statement (ii) is true. The blue color of the sky is caused by the scattering of sunlight by air molecules and other particles in the atmosphere (Rayleigh scattering). At very high altitudes, the density of the atmosphere is significantly lower, meaning there are far fewer particles to scatter sunlight. Therefore, scattering is not prominent. - The reduced scattering at high altitudes (ii) directly causes the sky to appear dark (i). Sunlight passes through the thin atmosphere with minimal scattering, so the observer does not see scattered light from the air in all directions. Instead, the unscattered light from the Sun and stars is visible against a dark background.
The scattering of a beam of light by colloidal particles is termed as:
Asked in: UPSC Geoscientist 2020
1. Thomson effect
2. Raman effect
3. Tyndall effect
4. Compton effect
Show answer
Answer: C. Tyndall effect
The Tyndall effect is the scattering of light as a light beam passes through a colloid. The individual suspension particles scatter and reflect light, making the beam visible. This effect is exhibited by colloids and fine suspensions, but not by true solutions.
Which one of the following waves has the longest wavelength?
Asked in: UPSC Geoscientist 2021
1. Visible light
2. Ultraviolet radiation
3. Infra-red radiation
4. X-rays
Show answer
Answer: C. Infra-red radiation
Wavelength and frequency are inversely proportional for electromagnetic waves traveling at the speed of light (c = fλ). Longer wavelength corresponds to lower frequency, and shorter wavelength corresponds to higher frequency.
Which one of the following statements is true about the appearance of colour of the Sun in the sky?
Asked in: UPSC Geoscientist 2022
1. At sunset (or sunrise), sunlight travels more distance in the atmosphere and higher frequency radiations scatter away resulting into red sunset (or sunrise).
2. At sunset (or sunrise), sunlight travels least distance in the atmosphere and higher frequency radiations scatter away resulting into red sunset (or sunrise).
3. At noon, sunlight travels least distance in the atmosphere and relatively less amount of sunlight is scattered and therefore the Sun appears reddish.
4. At noon, sunlight travels least distance in the atmosphere and larger amount of sunlight is scattered and therefore the Sun appears reddish.
Show answer
Answer: A. At sunset (or sunrise), sunlight travels more distance in the atmosphere and higher frequency radiations scatter away resulting into red sunset (or sunrise).
The color of the sky and the Sun's appearance depend on how sunlight is scattered by molecules in the atmosphere. Rayleigh scattering dictates that scattering is inversely proportional to the fourth power of the wavelength (scattering ∝ 1/λ⁴). This means shorter wavelengths scatter significantly more than longer ones. The path length of sunlight through the atmosphere is the key factor explaining the difference between the Sun's appearance at noon (shortest path, less scattering) and at sunrise/sunset (longest path, more scattering of short wavelengths).

Reflection and Refraction at plane and spherical surfaces

A person standing in front of a mirror finds his image larger than himself. This implies that the mirror is
Asked in: BPSC Prelims Exam 67th Cancelled · Assistant Engineer (Civil) 2018 19 2018
1. concave
2. plane
3. convex
4. cylindrical with bulging side outwards
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Answer: A. concave
A concave mirror can produce a magnified image. If the object is placed between the pole (P) and the focal point (F) of a concave mirror, the image formed is virtual, erect, and magnified. If the object is placed between the focal point (F) and the center of curvature (C), the image formed is real, inverted, and magnified. A person standing in front of a mirror usually refers to viewing an erect image, which happens when they are very close to a concave mirror (within the focal length).
Twinkling of stars in clear sky during nighttime can be explained with
Asked in: BPSC Prelims Exam 67th Cancelled
1. refraction of light
2. reflection of light
3. polarization of light
4. interference of light
Show answer
Answer: A. refraction of light
Twinkling (scintillation) of stars is caused by the repeated refraction of starlight as it passes through the Earth's atmosphere. The atmosphere has layers of varying densities and temperatures, which cause random fluctuations in the refractive index. This leads to continuous changes in the apparent position and brightness of the star as the light path bends slightly differently each moment.
The speed of light will be minimum while passing through
Asked in: BPSC Prelims Exam 67th Re Exam
1. vacuum
2. air
3. glass
4. water
Show answer
Answer: C. glass
The speed of light is maximum in a vacuum and decreases when it passes through a medium. The speed is inversely proportional to the refractive index of the medium. Among the given options, glass typically has the highest refractive index
Large number of thin strips of black paint are made on the surface of a convex lens of focal length 20 cm to catch the image of a white horse. The image will be
Asked in: BPSC Prelims Exam 68th
1. a zebra of black stripes
2. a horse of black stripes
3. a horse of less brightness
4. More than one of the above
Show answer
Answer: C. a horse of less brightness
Blocking part of a lens reduces the intensity (brightness) of the image but does not eliminate parts of the image or introduce patterns corresponding to the blocked areas.
The image formed by concave mirror is real, inverted and of the same size as that of the object. The position of the object should be
Asked in: BPSC Prelims Exam 69th
1. at the focus
2. at the centre of curvature
3. between the focus and centre of curvature
4. beyond the centre of curvature
Show answer
Answer: B. at the centre of curvature
This is a specific standard case in ray optics involving concave mirrors.
Which one of the following is not true about the image formed by a plane mirror?
Asked in: UPSC CDS 1 2020
1. It is of the same size as the subject.
2. It is laterally inverted.
3. It is real image.
4. It is formed as far behind the mirror as the object is in front.
Show answer
Answer: C. It is real image.
- Plane mirrors form virtual images. - Virtual images cannot be projected onto a screen. - Real images are formed by converging rays and can be projected.
In a periscope, the two plane mirrors are kept
Asked in: UPSC CDS 1 2020
1. parallel to each other
2. perpendicular to each other
3. at an angle of 60° with each other
4. at an angle of 45° with each other
Show answer
Answer: A. parallel to each other
- A simple periscope uses two plane mirrors. - The mirrors are placed parallel to each other. - Each mirror is tilted at 45 degrees to the axis of the periscope.
If the speed of light in air is $3 \times 10^8$ m/s, then the speed of light in a medium of refractive index $\frac{3}{2}$ is
Asked in: UPSC CDS 1 2020
1. $2 \times 10^8$ m/s
2. $\frac{9}{4} \times 10^8$ m/s
3. $\frac{3}{2} \times 10^8$ m/s
4. $3 \times 10^8$ m/s
Show answer
Answer: A. $2 \times 10^8$ m/s
- Refractive index $n = c/v$. - $c$ is the speed of light in vacuum (or air). - $v$ is the speed of light in the medium.

Thin and Thick Lenses, lens combinations

What type of lens is used in magnifying glass?
Asked in: BPSC Prelims Exam 67th Re Exam
1. Plano-concave lens
2. Convex lens
3. Convex mirror
4. Concave lens
Show answer
Answer: B. Convex lens
The type of lens used in a magnifying glass is determined by its ability to produce a magnified, virtual, and erect image. A convex lens does this when the object is placed within its focal length.
A myopic person has a power of -1.25 Dioptre. What is the focal length and nature of his lens ?
Asked in: UPSC CDS 1 2016
1. 50 cm and convex lens
2. 80 cm and convex lens
3. 50 cm and concave lens
4. 80 cm and concave lens
Show answer
Answer: D. 80 cm and concave lens
- The power of a lens (P) is related to its focal length (f) by the formula P = 1/f, where f is in meters and P is in Dioptres. - Given P = -1.25 D. - meters. - Converting to centimeters: -0.8 meters 100 cm/meter = -80 cm. - A negative focal length indicates a concave lens. Myopia (nearsightedness) is corrected using concave lenses to diverge light rays before they reach the eye's lens.
If the magnification produced by a lens is +2, then the image is :
Asked in: UPSC CDS 1 2022
1. erect, virtual and smaller than the object.
2. inverted, real and smaller than the object.
3. erect, virtual and larger than the object.
4. inverted, real and larger than the object.
Show answer
Answer: C. erect, virtual and larger than the object.
A positive magnification (+m) indicates that the image is erect (upright) relative to the object. A negative magnification (-m) indicates that the image is inverted. The magnitude of the magnification (| m|) indicates the size of the image relative to the object: |m| > 1 means the image is larger, |m| = 1 means it's the same size, and |m| 1). For a single lens, an erect image is always a virtual image.
A convex lens has a focal length of 15 cm. At what distance should an object be placed in front of the lens to get a real image of the same size of the object ?
Asked in: UPSC CDS 1 2022
1. 15 cm
2. 10 cm
3. 30 cm
4. 40 cm
Show answer
Answer: C. 30 cm
For a convex lens, a real image of the same size as the object is formed when the object is placed at a distance equal to twice the focal length (2f) from the lens. At this position (often referred to as 2F or C), the image is also formed at a distance of 2f on the opposite side of the lens, and it is real, inverted, and of the same size as the object. Given the focal length (f) is 15 cm, the object distance required is 2 f = 2 15 cm = 30 cm.
The power of a lens of focal length 10 cm is
Asked in: UPSC CDS 1 2023
1. 0⋅1 dioptre
2. 1 dioptre
3. 10 dioptre
4. 100 dioptre
Show answer
Answer: C. 10 dioptre
Power of a lens (in dioptres) = 1 / Focal length (in meters).
When a convex lens produces a real image of an object, the minimum distance between the object and image is equal to
Asked in: UPSC CDS 2 2018
1. the focal length of the convex lens
2. twice the focal length of the convex lens
3. four times the focal length of the convex lens
4. one half of the focal length of the convex lens
Show answer
Answer: C. four times the focal length of the convex lens
For a convex lens, a real image is formed when the object is placed outside the focal point (object distance |u| > f). The image formed is real and inverted. The lens formula is 1/v - 1/u = 1/f. Using distances as positive values, 1/v + 1/|u| = 1/f. Let D be the distance between the object and the image, D = |u| + v. To minimize D, we can express v in terms of |u| and f: 1/v = 1/f - 1/|u| = (|u|-f)/(f|u|), so v = f|u|/ (|u|-f). Thus, D = |u| + f|u|/(|u|-f) = (|u|(|u|-f) + f|u|)/(| u|-f) = (|u|² - f|u| + f|u|)/(|u|-f) = |u|²/(|u|-f). Let x = | u|-f, so |u| = x+f. D = (x+f)²/x = (x² + 2xf + f²)/x = x + 2f + f²/x. For a real image, |u| must be greater than f, so x > 0. By AM-GM inequality, x + f²/x ≥ 2√(x f²/x) = 2f. The minimum value occurs when x = f²/ x, i.e., x² = f², so x = f (since x>0). This means |u|-f = f, so |u| = 2f. When |u|=2f, v = f(2f)/(2f-f) = 2f²/f = 2f. The minimum distance D = |u| + v = 2f + 2f = 4f. This happens when the object is placed at 2f, forming a real image at 2f.
Which of the following lenses will bend the light rays through largest angle?
Asked in: UPSC CDS 2 2020
1. Lens with power +2·0 D
2. Lens with power +2·5 D
3. Lens with power –1·5 D
4. Lens with power –2·0 D
Show answer
Answer: B. Lens with power +2·5 D
The power of a lens (absolute value) indicates the extent to which it bends light; higher power means greater bending.
A luminous object is placed at a distance of 40 cm from a converging lens of focal length 25 cm. The image obtained in the screen is
Asked in: UPSC CDS 2 2020
1. erect and magnified
2. erect and smaller
3. inverted and magnified
4. inverted and smaller
Show answer
Answer: C. inverted and magnified
For a converging lens, when the object is placed between f and 2f, a real, inverted, and magnified image is formed beyond 2f. In this case, u = 40 cm, which is between f (25 cm) and 2f (50 cm).

Total Internal Reflection and applications

In total internal reflection, the light travels from
Asked in: UPSC CDS 2 2016
1. rarer to denser medium and it occurs with no loss of intensity
2. denser to rarer medium and it occurs with no loss of intensity
3. rarer to denser medium and it occurs with loss of intensity
4. denser to rarer medium and it occurs with loss of intensity
Show answer
Answer: B. denser to rarer medium and it occurs with no loss of intensity
For total internal reflection to occur, two conditions must be met: light must travel from a denser medium to a rarer medium, and the angle of incidence must exceed the critical angle. It is characterized by minimal to no loss of intensity upon reflection.
The appearance of a rainbow in the sky after a rain shower is due to
Asked in: UPSC Geoscientist 2022
1. diffraction and refraction of light in water droplets
2. total internal reflection of light in water droplets only
3. refraction of light in water droplets only
4. both total internal reflection and refraction of light in water droplets
Show answer
Answer: D. both total internal reflection and refraction of light in water droplets
When sunlight enters a rain droplet, it is refracted (bends) and dispersed (separated into different colors because different wavelengths refract at slightly different angles). This dispersed light then travels to the back inner surface of the droplet, where it undergoes internal reflection. Finally, as the light exits the droplet, it is refracted again, further separating the colors and sending them towards the observer's eye at specific angles depending on the wavelength, creating the arc of the rainbow. For the primary rainbow, the reflection at the back surface is usually total internal reflection, but even if not strictly TIR for all angles, it is an internal reflection that directs the light back towards the observer.
On a very hot day, we often see shimmering wavy lines near the ground. It is due to
Asked in: UPSC Geoscientist 2022
1. dispersion of light
2. refraction of light
3. reflection of light
4. total internal reflection of light
Show answer
Answer: B. refraction of light
On hot days, the ground heats the air just above it, creating a significant temperature gradient. Hot air is less dense than cooler air and has a slightly lower refractive index. Light rays passing through these layers of air with different refractive indices are continuously bent (refracted). As convection causes these pockets of hot and cool air to move and mix, the light rays are constantly being bent in different directions, making distant objects appear distorted, blurred, and wavy or shimmering. This is a form of atmospheric refraction.
Endoscope, used by doctors for examining the inside of a patient's stomach, works on the principle of
Asked in: JPSC CS Prelims 2011 · UPPCS Prelims 2010
1. reflection of light
2. dispersion of light
3. refraction of light
4. total internal reflection of light
Show answer
Answer: D. total internal reflection of light
The principle that enables light to travel along the optical fibers, even when they are bent, is Total Internal Reflection (TIR). TIR occurs when light strikes the boundary between two media at an angle greater than the critical angle, causing it to reflect back into the original medium.
Statement-I: Diamond is very bright. Statement-II: Diamond has very low refractive index.
Asked in: UPSC NDA 1 2015
1. Both the statements are individually true and Statement II is the correct explanation of Statement I
2. Both the statements are individually true but Statement II is not the correct explanation of Statement I
3. Statement I is true but Statement II is false
4. Statement I is false but Statement II is true
Show answer
Answer: C. Statement I is true but Statement II is false
Statement I is true because diamond is known for its exceptional brilliance and sparkle, which makes it a highly valued gemstone. Statement II is false; diamond actually has a very high refractive index (approximately 2.42), one of the highest among naturally occurring transparent minerals.
Optical fibres, though bent in any manner, allows light to pass through. What is the inference that one can draw from it ?
Asked in: UPSC NDA 1 2015
1. The concept that light travels in straight path is wrong
2. Light can flow through the optical fibres
3. Light can travel through the fibres because of their ductility
4. Light can travel through the fibres due to multiple total internal reflections
Show answer
Answer: D. Light can travel through the fibres due to multiple total internal reflections
Optical fibres consist of a core (higher refractive index) surrounded by cladding (lower refractive index). Light entering the core at a suitable angle undergoes total internal reflection at the interface between the core and cladding, bouncing back into the core. This process repeats along the length of the fibre, guiding the light even when the fibre is bent, as long as the bend is not too sharp.
An optical illusion which occurs mainly in deserts during hot summer is based on the principle of
Asked in: UPSC NDA 1 2017
1. Reflection
2. Interference
3. Dispersion
4. Total internal reflection
Show answer
Answer: D. Total internal reflection
- Mirages are caused by the bending of light (refraction) due to varying refractive indices in air layers of different temperatures. - Total internal reflection occurs when light travels from a denser medium to a less dense medium at a sufficiently large angle of incidence. - In hot conditions, air near the surface is hotter and less dense, creating conditions for total internal reflection of light traveling from cooler, denser air above.
Mirage is an illustration of
Asked in: UPSC NDA 1 2021
1. only dispersion of light.
2. only reflection of light.
3. only total internal reflection of light.
4. both refraction and total internal reflection of light.
Show answer
Answer: D. both refraction and total internal reflection of light.
Mirages are atmospheric optical illusions caused by the bending of light (refraction) as it passes through layers of air with different temperatures and thus different refractive indices. Near hot surfaces (like roads or sand in deserts), the air is hotter and less dense than the air above, meaning its refractive index is lower. Light from the sky or distant objects travels downwards into these layers of increasingly rarer (hotter) air. It bends away from the normal. If the angle of incidence becomes greater than the critical angle at the boundary between two air layers, total internal reflection occurs. This reflected light travels upwards to the observer's eye, creating the illusion of an image displaced from the actual object's position.

Wave Nature of Light: Huygens' Principle

The wavelength of visible spectrum is in the range
Asked in: BPSC Prelims Exam 63rd
1. 1300 Å-3900 Å
2. 3900 Å-7600 Å
3. 7800 Å-8200 Å
4. 8500 Å-9800 Å
Show answer
Answer: B. 3900 Å-7600 Å
- The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. - It is commonly defined as the range of wavelengths from about 380 to 750 nanometers (nm). - 1 Angstrom (Å) = 0.1 nanometer (nm), or 1 nm = 10 Å. - Therefore, the range in Angstroms is approximately: - Lower limit: 380 nm = 380 10 Å = 3800 Å. - Upper limit: 750 nm = 750 10 Å = 7500 Å. - The option 3900 Å-7600 Å falls well within this typical range and is often cited as the approximate limits.
Choose the correct statement.
Asked in: BPSC Prelims Exam 64th
1. Wavelength of red light is less than violet light.
2. Wavelength of red light is more than violet light.
3. Wavelength of violet light is more than green light.
4. Wavelength of violet light is more than yellow light.
Show answer
Answer: B. Wavelength of red light is more than violet light.
The visible light spectrum ranges from red to violet. Red light has the longest wavelength (approximately 620-750 nm), while violet light has the shortest wavelength (approximately 380-450 nm). The order of wavelengths from longest to shortest is Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV).
Which of the following does not change when light travels from one medium to another?
Asked in: BPSC Prelims Exam 66th
1. Velocity
2. Wavelength
3. Frequency
4. Refractive index
Show answer
Answer: C. Frequency
The relationship between velocity (v), frequency (f), and wavelength ($\lambda$) is $v = f\lambda$. When light changes medium, $v$ and $\lambda$ change, but $f$ remains constant.
A photon of X-ray has energy of 1 keV. A photon of visible radiation has energy of 3 eV. In this context, which one of the following statements is not correct?
Asked in: UPSC CDS 1 2017
1. The wavelength of X-ray photon is less than the wavelength of visible radiation photon.
2. Both the photons have different energies.
3. The speeds of both the photons in vacuum are different.
4. The frequency of X-ray photon is higher than the frequency of visible radiation photon.
Show answer
Answer: C. The speeds of both the photons in vacuum are different.
- The energy of the X-ray photon (1 keV = 1000 eV) is significantly higher than the energy of the visible radiation photon (3 eV). - Statement A: Since E = hc/λ, higher energy means shorter wavelength. E(X-ray) > E(Visible), so λ(X-ray) E(Visible), so f(X-ray) > f(Visible). This statement is correct.
Which one of the following is not a property of the X-rays?
Asked in: UPSC CDS 1 2020
1. They are deflected by electric fields.
2. They are not deflected by magnetic fields.
3. They have high penetration length in matter.
4. Their wavelength is much smaller than that of visible light.
Show answer
Answer: A. They are deflected by electric fields.
- X-rays are electromagnetic waves. - Electromagnetic waves are not deflected by static electric or magnetic fields. - X-rays have high penetration power and short wavelengths compared to visible light.
Infrared, visible and ultraviolet radiations/ light have different properties. Which one of the following statements related to these radiations/ light is not correct?
Asked in: UPSC CDS 2 2017
1. The wavelength of infrared is more than that of ultraviolet radiation.
2. The wavelength of ultraviolet is smaller than that of visible light.
3. The photon energy of visible light is more than that of infrared light.
4. The photon energy of ultraviolet is lesser than that of visible light.
Show answer
Answer: D. The photon energy of ultraviolet is lesser than that of visible light.
Electromagnetic radiations are ordered by their frequency or wavelength. In order of increasing energy (and frequency, and decreasing wavelength), the spectrum goes from Infrared (IR) to Visible light to Ultraviolet (UV). Photon energy is directly proportional to frequency (E = hf) and inversely proportional to wavelength (E = hc/λ). Therefore, UV radiation has higher frequency, higher photon energy, and shorter wavelength compared to visible light and infrared radiation.
The visible portion of the electromagnetic spectrum is
Asked in: UPSC CDS 2 2018
1. infrared
2. radiowave
3. microwave
4. light
Show answer
Answer: D. light
The electromagnetic spectrum encompasses a wide range of wavelengths, from radio waves to gamma rays. The visible portion of this spectrum, which is perceived by the human eye, is commonly referred to as visible light. This range includes the colors of the rainbow, from red (longer wavelengths) to violet (shorter wavelengths).
Which one of the following experiments explains the wave nature of electromagnetic radiation ?
Asked in: UPSC Geoscientist 2024
1. Black-body radiation
2. Photoelectric effect
3. Rutherford’s scattering experiment
4. Diffraction of light
Show answer
Answer: D. Diffraction of light
Diffraction occurs when waves bend around obstacles or spread out through openings. This bending and spreading are characteristic wave behaviors.

Fiber Optics: Principles and applications

Holography

Interference: Young's double-slit experiment, thin films, Newton's Rings

Laser: Principles and applications

Optical Instruments: microscopes, telescopes, human eye

Polarization of light: Brewster's Law, double refraction, optical activity

Prisms and Dispersion

Reflection and Refraction at plane and spherical surfaces

Thin and Thick Lenses, lens combinations

Total Internal Reflection and applications

Wave Nature of Light: Huygens' Principle

7. Heat and Thermodynamics

Practice MCQs: Optics

Diffraction: Fraunhofer and Fresnel diffraction, diffraction grating

Fiber Optics: Principles and applications

Holography

Interference: Young's double-slit experiment, thin films, Newton's Rings

Laser: Principles and applications

Optical Instruments: microscopes, telescopes, human eye

Polarization of light: Brewster's Law, double refraction, optical activity

Prisms and Dispersion

Reflection and Refraction at plane and spherical surfaces

Thin and Thick Lenses, lens combinations

Total Internal Reflection and applications

Wave Nature of Light: Huygens' Principle

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