# Q & A: light rays, energy and mass

Q:
Well i dont know whether i am asking a stupid question or not... But i need a serious answer for this... We all know waves and Rays ( ex :- radio waves, gama , x - rays ) travel in a massive speed.. Yet they doesn't have a mass because they are not made of matter.. But they carry energy.. And if we are to measure the Kinetic energy of them with 1/2mv^2 what do we take as the mass? If they really don't have a mass which means m = 0 (zero) the whole answer becomes 0 right? So they doesn't have an energy at all... Am I right or can someone describe me what's really right in this case? :)
- Nik (age 18)
Colombo , Sri Lanka.
A:

We'll point you toward old answers: .

For starters, though, mv2/2= p2/2m (where p is momentum) isnt even close to the right expression for energy for a particle that's moving fast in your reference frame. It just works for v << c. For particles traveling at c, the correct expression is pc. The full expression for the energy, working for all speeds, is sqrt(p2c2+mc4), where m is the rest mass.

Mike W.

(published on 04/26/2015)