Beginning through the Pedrotti optics book -- I have an energy question

[universal2013]

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!

 

[Doc Al]

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)?

As far as relativistic mass goes, it's rarely used these days. And that's not an expression for kinetic energy.

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 .

That equation is the relativistic energy-momentum relationship. Start here: Energy–momentum relation