Light passing through an optically transparent medium undergoes several physical changes due to its interaction with the particles constituting the media. Scattering takes place when the wavelengths of interacting particles are equal to or smaller than the wavelength of the incident light. Different types of scattering results depending upon the wavelength of incident light.
Wavelength equal to particle size
Majority of the scattered radiation results when wavelength of scattered light is same as that of incident light. Such scattered light is referred to as Rayleigh scattering. It results from elastic collisions between photons and the particles when no exchange of energy takes place
Wavelength less than size of interacting particles
A small fraction (1 in \(10^7\)) of scattered radiation has different wavelength from the incident light. This scattering is referred to as Raman scattering and results in Raman effect due to inelastic collisions between the photons and the particles. It involves a change in energy. Such energy changes result in. transitions between different vibration levels of molecules. The Raman lines to the left of the Rayleigh line have lower wavelength and are called Stokes lines and those to the right are called the anti-Stokes lines. Stokes lines have lower energy while anti-Stokes lines have higher energy than the Rayleigh peak.
Sir CV Raman discovered the Raman effect in 1928 and was awarded the Nobel prize in physics in the year 1930. Raman spectra are depicted in wave number units like the IR spectrum. The shift positions are conveniently expressed in terms of wave numbers.
Around 99.999% of incident photons undergo elastic collisions resulting in Rayleigh scattering. In Raman spectroscopy such scattered light is of no significance. It is only the 0.001% of incident light undergoing inelastic collisions that results in Stokes and anti-Stokes lines that are Raman active. A subsequent article will deal with techniques adopted to enhance very weak Raman signals in presence of high the intensity stray light resulting from Rayleigh scattering.