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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?
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/c2
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.
(published on 02/03/09)
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