Uh, E = mc^2 is not the full equation. That is, in fact, a special case for an object with no momentum in a given frame. The actual equation is E^2 = (mc^2)^2

Uh, E = mc^2 is not the full equation. That is, in fact, a special case for an object with no momentum in a given frame. The actual equation is E^2 = (mc^2)^2 + (pc)^2, where the p is momentum. And, unsurprisingly, the old "p=mv" isn't the whole story, either. The (magnitude of the) momentum of a photon is given by h/λ where h is Planck's constant and λ, of course, is the wavelength. As such, at no point is it necessary for light to have mass and it is believed that, in fact, photons are entirely and truly massless. Relativistic mass--which some take to mean photons actually gain mass--is a confusing and misleading concept that doesn't really have a whole lot to do with actual mass and doesn't really help explain anything. "Effective mass"--which is more related to stopping photons, but not really related to the Einstein equation you gave--also doesn't really mean the photon gains mass but it does mean that interactions within the crystal give effects similar to if the photon had mass.