Does light have mass? Why? 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?
 A: 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
A: 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.
