Have you ever wondered why is the sky blue? This article will explain why the sky is blue with the aid of a simple animation. If you look at the sky from the ground on a clear day, the sun will appear to be yellow and the sky around the sun will appear to be blue. The blue sky also gets paler as you get nearer to the horizon. If you were on the surface of the moon, the sun would appear to be pure white and the space around, black. Why is this?
The difference between the Earth and the Moon is that the Earth has an atmosphere. The effect that the air has on the appearance of the sun and sky can be attributed to a phenomenon called Rayleigh Scattering, where the extent of scattering of light is greater for light with a shorter wavelength.
Visible sunlight is made up of a mixture of colours, all the colours that can be seen in a rainbow (red, orange, yellow, green, blue, violet and indigo). When all these colours are mixed together we get pure white light. This is the sunlight that we would see if we were in space or on the Moon.
Light travels as a wave. Each colour has a different wavelength with red having the longest wavelength and indigo having the shortest. When the light passes through the atmosphere, the air scatters some of the sunlight. The shorter the wavelength, the more the light is scattered. This means that indigo, violet, blue and green are scattered more than red, orange and yellow. When the direct sunlight hits the ground, enough of the shorter wavelength colours have been filtered out by the scattering effect to change the apparent colour of the sun from pure white to yellow.
This accounts for why the sun appears to be yellow. But why is the sky blue?
The colours that were scattered by the air molecules will be radiated out in all directions with the same colours. In effect, the whole sky will glow with a mixture of the same colours that were scattered, i.e. more of the shorter wavelength light (indigo, violet, blue and green). Sky blue is actually a mixture of these short wavelength colours rather than a pure blue.
This explains why the sky is blue, but why does the sky get paler as you approach the horizon?
The sky blue light that travels to your eye from the horizon has to travel though much more air than that directly above you. As the sky blue light passes through this extra air it will in turn get scattered again resulting in less of the blue light reaching your eyes, making the light appear a paler colour.
On the next page, discover why the Sun looks red at sunset.