Well, well, well! It is a classical physics discussion BUT I THINK I WILL HAVE to bring special relativity in the case. Actually apparent mass=m/square root of 1-(v^2/c^2). Then if light will have mass, it will defy the special relativity. That means light when travelling at the velocity c will have infinite apparent mass which u can find by plugging the numbers in the above mentioned equation. So that is the fact behind it, u can also consult some postulates of Einstein's special theory of relativity to find a more convincing answer.
