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I've been wondering whether light has mass. Yet given the wave-particle duality of light, the statement seems to be affirmative. With that, how to calculate it?

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In quantum field theory, a photon's rest mass is proven to be zero. But relativistically, the photon's energy leads to the relativistic mass $m=\frac{h\nu}{c^2}$.

Related link: http://www.desy.de/user/projects/Physics/Relativity/SR/light_mass.html

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I would avoid mass concept of photon at all, because it doesn't have rest mass. Relativistic mass is very slippery thing and is not unambiguously defined. Someone just put Lorentz factor $$ {\frac {1}{\sqrt {1-{\frac {v^{2}}{c^{2}}}}}} $$ besides rest mass (multiply by $m_0$) and has called it "relativistic mass". Now we know for sure that photon has NO rest mass, so you can't substitute something for $m_0$ in photon case. Thus photon doesn't have relativistic mass too !

However, it has momentum: $$p = mc$$ using Einstein famous relation between mass and energy $E=mc^2$ and photon energy $E=h\nu $, we get momentum as: $$ p = \frac{h\nu}{c} $$

Still a bit strange that some object without a rest mass can have momentum, but it is so !
There are numerous experiments which proves light pressure. To me photon and fields in general are very strange things.

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