Hello. I started to work on pedrotti optics book (2nd edition) and i got confused about what is relativistic mass and why we use it rather than kinetic energy (1/2mc^2)? Also in the beginning of these explanations there is one equation i barely understand nothing out of it. Could you please explain this equation and where does it comes from please? P=sqr(E^2-(mc^2)^2/c . Thank you!