Technique based on a light scattering effect, named after the Indian physicist, C. V. Raman, who was the first to observe it in 1928 following its theoretical prediction by A. Smekal. It is easiest to appreciate the scattering process by thinking of the monochromatic light wave of frequency v as a stream of particles, called photons, each of which carries a packet of energy hv. If this beam of photons is directed at a molocule, it will in general bounce off it (provided the frequency v does not correspond to an absorption frequency of the molecule). The great majority of photons will be scattered in all directions but without change of energy. This process is called elastic scattering and was well understood by nineteenth century physicists like Lord Rayleigh; indeed it is referred to as Rayleigh scattering. A very small of the photons manage to exchange some energy with the molecule as they interact with it; this is inelastic or Raman scattering.