White light from the sun is ‘scattered’ as it passes through the atmosphere. White light is a spectrum of many different colours; red, orange, yellow, green, blue, indigo and violet. The amount of scattering depends on the wavelength of light. This effect is called Rayleigh scattering, which is named after Lord Rayleigh who discovered it first.

Blue and violet colours, have shorter wavelengths hence they are ‘scattered’ more than others. Our eyes are more sensitive to blue light than to violet light hence the sky appears blue.

Blue Sky

The ‘scattering’ of white light in the atmosphere is caused by the components of air (mostly nitrogen and oxygen). The colour blue fades towards the horizon when the sun is near the horizon at dawn and dusk, this is because, the rays from the sun strike the atmosphere at oblique angles thereby making them travel a greater distance through the atmosphere than they would at midday. Hence, the shorter blue and violet wavelengths are filtered out leaving longer wavelengths (red, orange and yellow) which are seen during the sunrise and the sunset.