Tyndall effect or Rayleigh scattering, is concerned with the intensity of scattered light that depends on the fourth power of the frequency, so blue light is scattered much more strongly than red light. With our sky especially on a sunny day, it appears blue when sunlight bounces off small particles in the atmosphere. At first, physicists such as Lord Rayleigh and John Tyndall thought that the blue color of the sky was due to small particles of dust and droplets of water vapor in the atmosphere.

Later they realized that if this were true, there would be more variation of sky color with humidity or haze conditions than was actually observed. So they supposed correctly that the molecules of oxygen and nitrogen in the air were sufficient to account for the scattering.

The case was finally settled by Einstein in 1911, who calculated a detailed formula for the scattering of light from molecules. He was able to use his calculation as a further verification of Avogadro's number when compared with observation. Molecules are able to scatter light because the electromagnetic field of the light waves induces electric dipole moments in the molecules.

The Rayleigh scattering is clearly seen when light-scattering particulate-matter is dispersed in an otherwise light-transmitting medium and when the cross-section of an individual particulate is the range of roughly between 40 and 900 nanometers.

More Info: en.wikipedia.org