E = mc2 applies to all mass and energy
Energy goes into heat when it is absorbed. Heat causes end of box to get (slightly) heavier!
Applies to ALL kinds of energy
In our ordinary life, the change of mass usually too small to detect (because c is so large!)
E = mc2 -----> 1 Kg x (3 x 108)2 = 9 x 1016 Joules 1 Joule = 10-17 Kg
About 1Kg mass = entire energy output of large power plant in one year
Light transmits energy (mass) from
one end of box to the other