Posted on 17 March 2011 at 13:05h

Just been doing scattering with my HL class and was getting myself confused trying to explain the blue sky in the day compared to the red sky at sunset. I think I might have been mixing up to different effects, not that we have many sunsets in Norway at this time of year.

Firstly some facts about scattering, properly named Rayleigh scattering which is the scattering of light by small particles such as molecules in the air. The Intensity of light scattered at an angle ϑ is inversely proportional to λ⁴ so at large angles the intensity of shorter wavelengths will be greater. So when looking at the sky in the daytime you are looking at the light that is scattered at large angles from the suns rays.

The amount of light scattered increases as the light passes through more atmosphere so at sunset when the light passes through a lot of atmosphere more of the blue light will have been scattered than at midday, the light directly from the sun has no blue so the sun looks red. But what makes the sky look red? This is I think due to another effect; if there is a lot of pollution in the air the larger particles scatter the red light making the sky look red, this can also happen at midday if there is a lot of smoke in the atmosphere after a volcano or something. This effect is also made greater when the atmosphere is thicker hence is normally only seen at sunset (still not totally convinced by this last bit).

Hyperphysics