General Introduction. It is a special type of spectroscopy which deals not with the absorption of electromagnetic radiation but deals with the scattering of light by the molecules. It is observed that when a substance which may be gaseous, liquid or even solid is irradiated with monochromatic light of a definite frequent v, a small fraction of the light is scattered. Rayleigh found that if the scattered light is found to have the same frequency as that of the incident light. This type of scattering is called Rayleigh scattering.

Prof. C.V. Raman of Calcutta University, however, observed in 1928 that when a substance (gaseous, liquid or solid) is irradiated with monochromatic light of a definite frequency v, the light scattered at right angles to the incident light contained lines not only of the incident frequent but also of lower frequency and sometimes of higher frequency as well. The lines with lower frenzy are called stokes’ lines whereas lines with higher frequency are called anyi-Stokes’lines Raman further observed that the difference between the frequency of the incident light and that of a particular scattered line a was constant depending only upon the nature of the subsistence being irradiated and was completely independent of the frequency of the incident light. If  V_i is the frequency of the incident light and V_s that of a particular scattered line, the difference V=V_i -V_s is called Raman frenzy or Raman shifty. Thus the Raman frequencies observed for a particular substance are characteristic of that substance. The various observations thus made by Raman constitute what is called Raman effect and the spectrum observed is called Raman spectrum. Thus in a simple way. Raman spectrum may be represented as follows:
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