Rayleigh and Raman Scattering

For molecules, two types of scattering can occur. The first type of scattering, Rayleigh scattering, is an elastic scattering process in which a photon bounces off a molecule like a billiard ball, emerging with the same energy as it entered. The second type of scattering, Raman scattering, is an inelastic scattering process in which the light scattered by a molecule emerges having an energy that is slightly different (more or less) than the incident light. This energy difference is generally dependent on the chemical structure of the molecules involved in the scattering process, and emerges as a result of the same rotational and vibrational energy modes discussed here.

Most of the scattering that occurs in nature is Rayleigh scattering, and it is one of the most commonly observed optical phenomena. For example, Rayleigh scattering is the reason the sky looks blue: the intensity of the light that gets Rayleigh scattered by a molecule is inversely proportional to the fourth power of the wavelength of the incident light, which means that 10 times more blue light than red light gets scattered from a molecule. Hence, sunlight incident on gas molecules in the air gets scattered as blue light in every direction, and the sky looks blue. Comparatively, Raman scattering is much less prevalent. For approximately one million photons Rayleigh scattered by a molecule, only one photon is Raman scattered. Indeed, the most significant challenge in detecting Raman scattered light is to separate it from the predominant Rayleigh scattered light that accompanies it.

More technical details on the Raman scattering process and its applications can be found here.


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