Tyndall EffectJohn Tyndall (1859) discovered that when light passes through a clear fluid holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than the red. (also in scuba diving due to absorption)This is most correctly called the Tyndall effect but it is more commonly known to physicists as Rayleigh scattering after Lord Rayleigh who studied it in more detail a few years later. He showed that the amount of light scattered is inversely proportional to the fourth power of wavelength for sufficiently small particles. It follows that blue light is scattered more than red light by a factor of (700/400)4 ~= 10. Dust or Molecules?Tyndall and Rayleigh thought that the blue color of the sky must be due to small particles of dust and droplets of water vapour...but....Later scientists realized that if scattering were caused by dust and water there would be more variation of sky color with humidity or haze conditions than was observed so they supposed correctly that the molecules of oxygen and nitrogen in the air are sufficient to account for the scattering. The case was finally settled by Einstein in 1911 who calculated the detailed formula for the scattering of light from molecules which was found to be in agreement with experiment. He was even able to use the calculation as a further verification of Avogadro's number when compared with observation. The molecules are able to scatter light because the electromagnetic field of the light wave induces an electric dipole moment. Why not violet?If shorter wavelengths are scattered most strongly, why is the sky not violet (shortest wavelength)? |