begin mathsize 36px style lambda minus lambda apostrophe space equals space fraction numerator h over denominator m subscript e c end fraction open parentheses 1 minus space cos theta space close parentheses end style

λ= Initial wavelength of the photon before scattering; λ' = Final wavelength of the photon after scattering; θ = Scattering angle; h= Plank's constant ; me = Rest mass of an electron;   c = speed of light in vaccum 





When a photon hits an electron (or any quasi-free charged particle), the wavelength of the photon increases due to the energy loss of the photon. This is an inelastic scattering since the energy (so as the wavelength) of the scattered particle got changed.  This is called "Compton Scattering."

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