(notes by R. Bigoni)

A photon with energy and momentum collides with an electron initially at rest.

If we represent with *m _{0}* the mass of the electron at rest, with

We can eliminate the functions of the angle *ψ* by carrying it to the left hand side, squaring
both sides and adding the equations together:

From the principle of conservation of energy, using the relativistic expressions, we get

If we carry *m c ^{2}* to the left hand side and square both sides we have

Now we join together the obtained results

and subtract these equations side by side

The left hand side of this equation is equal to *m _{o}^{2}c^{4}*: infact

So we get

The constant

which is particularly interesting since it results from a combination of three fundamental physical constants,
is said **Compton wavelength**.

Finally, we can write

**Scattered photons have wavelengths longer than the incident ones, and these wavelengths depend
from the angle at which they are observed**.

This effect, observed in 1922 by A. Compton, in the analysis of the scattering of X-rays by a block of paraffin, was in contrast with the classical wave theory so it was one of the most important experimental facts for the development of quantum mechanics.

last revision: October 2015