[CZARCAR (OP)]

i dont get it. if mass=0, then photon has no energy?

[JP]

That's only the correct equation when you're calculating the energy of something that is sitting still.  Light always moves at light speed, so the correct equation in that case is:

E²=(mc²)²+(pc)²,

where p is momentum.  A photon has no mass (m=0), but it has momentum, (p≠0).  Therefore, it has energy.

[CZARCAR (OP)]

thanx