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The photon's energy is given by E = hv, where h is Planks constant and v is the frequency. It has momentum too, you just cannot use mass to calculate it. Its momentum is given by p = h/wavelength.EnumaElish said:That's a "wow." It's obvious when I think about it, yet... How does physics explain this? (I am sure the explanation is obvious to a physicist, too.)

The usual equation for mass m^2 = E^2/c^4 - p^2/c^2 is equal to zero in the case of the photon because the wavelength times the frequency equals the speed of light c.

Or in other words E = hv = hc/wavelength.

So the equation for mass becomes,

m^2 = (hc/wavelength)^2/c^4 - (h/wavelength)^2/c^2 = 0.

Here is an interesting link http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html [Broken]

Which discusses the question of whether a photon has mass.

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