Fermilab

Let us see some of the properties of photons.

• The energy of a photon related to an electromagnetic wave whose frequency is is given by
• We know light propagates with the speed of light. Therefore the speed of photons is c.
• The rest mass of a photon is 0.

We learned earlier that objects with nonzero rest mass cannot propagate with the speed of light. This comes from the fact, that if you calculate the dynamical mass for greater than 0 and v=c, you get nonsense, a diverging expression. It looks as if the dynamical mass became infinite, which means the energy of the particle became infinite. However, if the rest mass is zero, you get 0/0, which in mathematics means an arbitrary number. By using mathematical tricks (limits), one can still extract some information from such limits and get a meaningful value for the dynamical mass.

• The dynamical mass of the photon is

  

• The momentum of a photon is

  (just as in classical physics, momentum equals mass times speed) which gives us

This last item is very important. It sets the bridge between the particle and wave description of the world, as we will see on the next page.