What is Raman Effect?

Raman effect or Raman scattering is the inelastic scattering of photon (the quantum of light)

This effect was discovered by sir C.V. Raman; hence the name.

English: Raman and Rayleigh scattering process...

The elastic scattering, the commonly observed and most of the photons undergo this, is also called Rayleigh scattering. In this the scattered  photons have the same energy (frequency and wavelength) as the incident photons. But a small fraction of the scattered photons have a different wavelength and frequency. Normally such photons have less frequency (greater wavelength) than the incident photons.

The Raman spectrum of pure water

The Raman spectrum of pure water (Photo credit: The^Bob)