However, infrequent inelastic collisions can cause molecular excitations and change the photon's wavelength.  
Monochromatic photons traversing transparent media usually exhibit Rayleigh scattering caused by elastic photon-molecule collisions. However, infrequent inelastic collisions can cause molecular excitations and change the photon's wavelength.  Used with lasers to determine the composition and structure of chemical compounds this effect, for ten points, was discovered in 1928 and named for what Indian physicist?
Monochromatic photons traversing transparent media usually exhibit Rayleigh scattering caused by elastic photon-molecule collisions.
Raman effect or scattering