Why is the sunset coloured red?

Imagine you are sitting on a lonesome beach with your girlfriend and you are starring at a wonderful red sunset. But how does the sunset work? Why is it red-coloured although the normal light is white or yellow?

To touch the earth the sunlight has to pass several strata. The last strata is our atmosphere, filled with gases like oxygen, nitrogen and CO2 and many others in less concentrates. Each of these gases has got its own molecules that are flying to the atmosphere. When the light of the sun passes our atmosphere this light is scattered different directions at the molecules of the gases. White colour consists of many different colours (spectral colours) with different wavelength. From the short-waved blue light to the long-waved red light.

Because of the long wavelength of the blue light it is scattered more often than the red light. This is the cause of the blue colour of the sky. So blue is the "backlight" of the sky. Only the sun is coloured yellow, because it is light with a shorter wave-length.

In the evening the sun stand in a shorter angle to the earth. So the light has to go a long way through the atmosphere and the probability is much bigger that the light is scattered. So the long wavelength light like blue is scattered more and in the end there is only the red (short length light) left. This effect is effort by particles of dirt in the air. So if you have got a dirty sky the sunset will be more beautiful.

By the way, do you know how to build your own sunset?

For this experiment you need a strong pocket lamp, tequila and a glass with a thin and planar ground. Now you fill some of the tequila in the glass. Darken the room and put the lamp under the ground. Now fill some water (only some drops) in that glass (the tequila gets milky). During filling water in the glass look at the wall and you see a point turning from orange into a dark red. Now you have your own sunset!