A mirage is an optical phenomenon caused by the bending of light rays. This bending of light is known as refraction (3). Refraction occurs because light travels at different speeds through air layers of different densities. These different densities are created by temperature gradients in the atmosphere. Terrestrial heating (1), such as sunlight heating the ground or a road surface, causes the air near the surface to become much hotter and less dense than the air layers above. Light rays from the sky or distant objects pass through these layers of varying density, causing them to bend upwards (in the case of an inferior mirage), creating the illusion of a pool of water or a reflection. Reflection of light (2) and Diffraction of light (4) are not the primary physical phenomena responsible for the formation of a mirage itself, although the mirage is perceived as a 'reflection'.