5. Why is the sunset red?

Absorption spectroscopy has allowed us to understand why some objects appear the colour they do. Now we will move on to ask why the sky is blue and the sunset red? In order to understand this we need to consider the process of light scattering.

The scattering of light occurs by the interaction of light with molecules and small particles in the atmosphere – a process sometimes referred to a Tyndall or Rayleigh scattering. The intensity of scattering is inversely related to the wavelength of the light – blue light is scattered more than red.

To an observer standing on the earth with the sun in the sky, the sky around him appears blue because of scattering of the short wavelength light from the atmosphere. The photographs taken by the astronauts on the surface of the moon show a black sky –simply because there is no atmosphere there!

At sunset the light from the sun grazes across the surface of the earth, passing through a long column or pathlength of atmosphere. During its passage through the atmosphere the blue components of the light are reduced in intensity, making the transmitted beam of sunlight more yellow or even red in colour.

In the laboratory we can demonstrate scattering using a solution containing a suspension of very fine particles. Colloidal sulphur is generated by the acidification of an aqueous thiosulfate solution and the scattering of light through the solution observed.

A. Beeby, 04/09/08