CBSE Class 12-science Answered

"Because of the equivalence of energy and mass, the energy which an object has due to its motion will add to its mass." and Richard Feynman in "The Character of Physical Law" wrote "the energy associated with motion appears as an extra mass, so things get heavier when they move." The case of photons and other particles which move at the speed of light is special. From the formula relating relativistic mass to invariant mass, it follows that the invariant mass of a photon must be zero but the relativistic mass need not be. The phrase "The rest mass of a photon is zero" sounds nonsensical because the photon can never be at rest but this is just a misfortunate accident of terminology. In modern physics texts the term mass when unqualified means invariant mass and photons are said to be "massless"