The Raman effect

When light enters a material a fraction of the light is scattered in all directions. A tiny portion of the scattered light suffers slight shifts in frequency (color), which are characteristic of the material and independent of the incident light frequency.

The shift is due to an inelastic scattering of light in which the light exchanges energy with vibrations of the molecules in the material. It is called Raman effect after the Indian physicist C. V. Raman who discovered it. The frequency shifts depends on the chemical bonds and atoms that form the material.