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When a photon strikes an atom (say carbon atom in a graphite crystal), it may scatter from a loosely bound electron, which is essentially free. In this case there is a measurable shift in the wavelength of the scattered photon. It is also likely that the photon scatters from an electron that is tightly bound to an atom. In such a case, the mass appearing in Compton's formula must be replaced by the mass of the carbon atom itself, which is approximately 20,000 times heavier than an electron. The maximum wavelength shift of the photon for scattering from a free electron is twice the Compton wavelength of an electron, i.e.  nm. In case of scattering from the carbon atom, the maximum wavelength shift is approximately  nm, which is very small. Thus we find an intensity maximum at an wavelength which is essentially equal to that of the incident wavelength. |
