Q:

Taking the definition of mass to be the one that defines an object (in this case a photon) whith momentum as having a mass. What is the mass of a photon? Does this vary dependant on wavelength? IE is one colour 'heavier' than an other?

- Jamie

- Jamie

A:

Since the momentum is h/lambda (where h is Planck's constant and lambda is wavelength) the mass (in the sense you use the word) is just h/(c*lambda) or hf/c^{2} where f is the frequency and c is the speed of light.

It turns out that this 'mass' is what enters into gravitational attractions. So photons of different colors do indeed have different weights. If you had a huge bag of blue photons it would literally weigh more than a bag of the same number of red photons.

Mike W.

It turns out that this 'mass' is what enters into gravitational attractions. So photons of different colors do indeed have different weights. If you had a huge bag of blue photons it would literally weigh more than a bag of the same number of red photons.

Mike W.

*(published on 04/19/2008)*