The blue colour of the sky is primarily caused by the scattering of sunlight by molecules and tiny particles in the Earth's atmosphere. This phenomenon is explained by Rayleigh scattering, which states that the amount of scattering is inversely proportional to the fourth power of the wavelength. Shorter wavelengths (like blue and violet) are scattered much more effectively than longer wavelengths (like red and orange). When sunlight enters the atmosphere, the blue light is scattered in all directions by the atmospheric particles, making the sky appear blue. Our eyes are more sensitive to blue light than violet, contributing to the perception of a blue sky.