 # The equivalence of mass and energy expressed by the relation E = mc2 is a fairly direct consequence of Maxwell’s conception of electromagnetic waves. According to Maxwell’s equations, the energy density of electric and magnetic fields e and b is (e2 + b2)/(8p) and the momentum density is (e x b)/(4pc). Thus in the case of radiation (when e and b are equal and orthogonal) the energy density is E = e2/(4p) and the momentum density is p = e2/(4pc), so we have the important relation p = E/c. In addition, the energy of a pulse of light for relatively moving systems of reference transforms in proportion to its frequency, consistent with Planck’s relation E = hn. Given these facts, we can show that light pulses of total energy E reflecting inside a closed container contribute E/c2 to the inertial mass (i.e., the resistance to acceleration) of the container.this one I got from a site.Here it states that, electric field=magnetic field(for light) But in the ncert physics of class 12 and many other sites it is given thatE/c=BE=electric fieldB=Magnetic fieldc=speed of lightSo, which is correct 