Erin Norris, of Ilam Primary School, asks :-

Why does the sky change colour when the sun sets?

Graham Batchelor, a physicist at Riccarton High School, responded.

Red light can penetrate through the atmosphere quite well but blue light at the other end of the spectrum of colours has difficulty getting through the particles of the atmosphere. This is because blue light is scattered much more readily than red is. That is why the sky appears blue to us. The only light that reaches our eyes from the sky is sunlight which has been scattered towards our eyes by the earth's atmosphere.

When the sun is overhead only a small amount of its blue light is scattered and so it still appears to be white to us. However, when the sun is setting the light has to travel through much more atmosphere. Hence at sunset the white sun appears red because almost all of the blue has been scattered away from the direct line from the sun to your eyes whereas some red light arrives unscattered. This makes the setting sun less bright and it, and any objects it illuminates, such as clouds or mountain tops, appear red. One of Australia's great tourist attractions is to watch the changing colours of Ayres Rock during sunset.

You can see the same effect as this by looking at a distant white street light on a smoggy night. The white lights at the far end of the street appear to be reddish because the blue light from these lamps can not get through the smog particles unscattered but the red can.