Compton Effect (2 questions)

1. It is known that the Compton effect consists in the scattering of photons on to free electrons.

a) Which electrons of a substance can be approximated as being free?

b) Why in the case of visible radiation the Compton effect can not be observed?

2. Explain the following peculiarities of the Compton scattering:

a) The intensity of the scattered component of radiation increases with decreasing the atomic number of substance and also increases with increasing the scattering angle.

b) The value of the scattering does not depend on the nature of the target substance.

c) Why there exists a non scattered component?


a) The “free” electrons are the electrons from atoms having ionization (“bonding”) energies less than the energy of the incoming photons (usually 10 times less than).

b) In the case of visible radiation the energy of the incoming photons is comparable with the ionization energy on the electrons, so that in this case the scattering does not happen.


a) In both cases the increasing of the intensity of the scattered component is due to the increasing of the number of free electrons available to scatter the photons. In the light atoms all the electrons are weakly bonded (they have low ionization energies), whereas in the heavy atoms only the outer electrons (from the last orbit) are weakly bonded.

Small scattering angles correspond to collisions of the incoming photons with strong bonded electrons, whereas the large scattering angles corresponds to collisions of photons with weak bonded or free electrons.

b) This happens because the scattering takes place on to free electrons, which are “universal” particles, so that the nature of the substance is excluded.

c) The non scattered component is due to the collisions of photons with atom nuclei or with strong bonded electrons (inner electrons).