How do you calculate the momentum of a photon?

1 Answer
Mar 22, 2018

Answer:

The momentum of a photon is given by #E/c hat n# where #hat n# is the unit vector in the direction in which the photon is moving, #E# is its energy, and #c# is the speed of light.

Explanation:

For any particle of mass #m# (this is the so called invariant mass of the particle, often called the rest mass and denoted by #m_0#), the energy and the momentum are related by

#E^2-|vec p|^2c^2 = m^2c^4#

For the photon, the mass #m# is zero and this leads to

#|vec p| = E/c#

Note that the formula
#vec p = {m vec v}/sqrt{1-v^2/c^2}#
which is valid for massive particles, fails for the photon since both #m# and #1-v^2/c^2# vanish for it.