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When a strong beam of visible or UV light illuminates a gas a liquid or transparent solid a small fraction of light is scattered in all directions the spectrum of the scattered light is found to consist of lines of the same frequencies as the incident beam called Rayleigh lines and also a certain weak lines of changed frequencies. These additional lines or weak lines are called Raman Lines.

The lines on the two frequency side are called Stokes Lines while those on the high frequency side are called Anti-stokes lines.
The anti-stokes lines are much weaker than stokes lines this phenomenon is called Raman effect.

Fig. Raman Spectrum

EXPERIMENTAL SETUP :

The basic requirement for Roman spectrum are a source a roman tube and a spectograph.

The source must be an intense line source in blue voilet region.
A mercury arc is a proper source nowadays LASER provides on exceptionally intense and mono-chromotic raman source
The raman tube used for liquids is a thin glass tube T whose one end is closed with on optically plane glass plate and the other is down into the slope of horn and covered with black tape the flat end serve as the window through which the scattered light energy from horn shape and causes total reflection of the backward scattered light and provides a dark background.

The spectrograph must be one of high light gathering power combine with good resolution. The Raman tube T containing experimental liquid is placed above and parallel to the source S.

In between the source and tube there is a glass cylinder placed which is filled with saturated solution of sodium nitrate.
The sodium nitrate solution absorb the UV light of the mercury arc but transits the blue light with greater intensity.

A polished reflector R placed over T increases the intensity of illumination
The scattered light passing through the plane window of the Raman tube is focused on the slit of a spectrograph which phtograph the specturm.

Fig. Experimental Setup 

EXPLANATION:

The Raman effect can be explained from Quantum theory according to this theory light of frequency is a bundle of photons each of energy hv.

When it falls on a scatterer the photons collides with the molecules of the scatterer.

There are three possibilities in such a collision :

  • The photon may be scattered without loss or gain of energy it then gives rise to the unmodified spectral lines of the same frequency as of the incident light this is REYLIEGH LINES.

  • The photon may give a part of its energy deltaE to a molecules which is a great energy states E the molecules is that excited to a higher energy state and the photon is consequently scattered with a smaller energy. In this case it gives rise to a spectral line of lower frequency or longer wavelength. These Is STOKE’S LAW.
  • The photon may collide a  molecules already in excited state E2 and take on energy from it in this case the molecules is de-excited to the ground state E1 and the photon is scattered with increased energy HV + DeltaE It give rise to spectral line of higher frequency this is Antistokes line’s.
Fig. Stokes & antistokes lines
Since the number if molecules in excited state is very small the chances of the process are very small hence Antistoke’s Raman lines are much weaker than the Stoke’s Raman lines.