Convincing evidence that light is made up of particles (photons), and that photons have momentum, can be seen when a photon with energy hf collides with a stationary electron. Some of the energy and momentum is transferred to the electron (this is known as the Compton effect), but both energy and momentum are conserved in this elastic collision. After the collision the photon has energy hf^/ and the electron has acquired a kinetic energy K.

Conservation of energy: hf = hf^/ + K

Combining this with the momentum conservation equations, it can be shown that the wavelength of the outgoing photon is related to the wavelength of the incident photon by the equation:

Dl

=

l/ - l

=

h me c

(1 - cosq)

where m_e is the mass of the electron.

The combination of factors

h me c

= 2.43 x 10-12 m

is known as the Compton wavelength.

The collision causes the photon wavelength to increase by somewhere between 0 (for a scattering angle of 0°) and twice the Compton wavelength (for a scattering angle of 180°).