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Unformatted text preview: all p if m 2 c 4 = 0, that is, m = 0. Light is in fact composed of photonsparticles having zero rest mass, as we shall discuss later. The rest mass of a photon is meaningless, since theyre never at restthe energy of a photon is of the form 0/0, since m = 0 and v = c , so m can still be nonzero. That is to say, the mass of a photon is really all K.E. mass. For very fast electrons, such as those produced in high energy accelerators, the additional K.E. mass can be thousands of times the rest mass. For these particles, we can neglect the rest mass and take E = cp ....
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