By Sunil Bhardwaj

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When molecular sample is placed in an intense beam of monochromatic radiations, it transmit the entire incident light. In this case molecule gets excited and immediately returned to ground state. The returning molecule takes place with emission of light in all the directions. This type of scattering of light is known as Rayleigh scattering.

In 1923, smekel theoretically predicted that if the substance in solid liquid or gaseous state is irradiated with monochromatic light, the scattered light must contain radiations with the different frequencies than that of incident.

In 1928, Sir C.V.Raman found that the scattered light must contain some lines which have the frequency more than incident and some lines possess the frequency less than incident. The lines which have frequency less than incident are called Stokes Lines and those which have more than incident are called Antistoke Lines. These stoke and anti-stoke lines form Raman Spectrum.$$ \triangle \nu ={ \nu }_{ incident }-{ \nu }_{ scattered } $$

Polarisibility in a molecule

\(\triangle \nu\) is positive means \({ \nu }_{ incident }>{ \nu }_{ scattered }\) hence stoke lines.

\(\triangle \nu\) is negative means \({ \nu }_{ incident }<{ \nu }_{ scattered }\) hence Antistoke lines.

Polarisibility in a molecule