As to mass of light. M=M0/(1-v^2/c^2)^0.5, and for photon, if its speed in vacuum is c, and its still mass M0 is not zero, M will be infinite. And photon never stop, then we will be always in a world full of stuffs with infinite mass~~
Unitil now, M0 of photon is zero, and its speed in vacuum is a constant c, then we cannot get what its "mass in motion" is by M=M0/(1-v^2/c^2)^0.5, coz it will be a 0/0. The formula we need to use is E=hv, and E=Mc^2, then M=hv/c^2, then p=hv/c, and this is
the expression of momentum of photon as we all know.
Actually the term "mass" is meaningless. It's not an objective reality, but an "artificial" definition. Eg. in general grativity, there is no clear definition of "mass" coz it's not important. The term "Energy" and "Momentum" are comparatively objective and
And, the fact that light is under the effect of gravitational field doesn't necessarily mean that it must have a still mass, coz the term "mass" can be defined (if we really want to) as the ability to generate gravitational field, or the ability to curve the
space-time. Photon does't generate any gravitational field, but it can be affected by gravitational field like everything in the universe. The classic Newton formula of F=mv^2/r and gravitation formula of F=GMm/r^2 are still applicable to photon, and that's
how we got the escape velocity formula of v=(2G(M+m)/r)^0.5, then we can see, m=0 wont affect the validity of this formular. And, that's how Schwazschild radius formula came out: R=2GM/c^2.