Scattering of light

We observe that the sky appears blue, there is a reddening of sky at sunrise and at sunset. The path of beam of light becomes visible when it passes through space containing smoke, tiny water droplets, suspended dust particles. The path of light is visible when it pass through a medium the size of whose particle are comparable to wavelength of light.

Colour of scattered light depends upon the size of scattering particles. Very small particles scatter blue light while larger particles scatter light of longer wavelength. The following experiment will make the point clear.
When a little of sulphuric acid is put in a dilute solution of sodium thiosulphate (hypo), sulphur particles begin to form. The particles grow slowly with time.
Experiment: Take a white source of light (S) at focus of converging lens L₁. The beam is rendered parallel. Allow it to pass through tank T containing pure water. Allow the beam to pass through a hole C and then converge to screen MN by a converging lens L₂. A sharp bright image will be formed on MN.
Dissolve about 100 g of hypo in a litre of pure water. Put it in tank T and add about 1 ml (1 cm³) of concentrated sulphuric acid in it. You will observe blue light from three sides of the glass tank. This is due to scattering of smaller wavelengths of light by minute colloidal particles of sulphur formed. However, in fourth side facing the circular hole C will first appear orange-red colour and then bright crimson red colour on the screen.