Mass of a photon
Many students have asked about the mass of the photon.
It all depends what you call mass. Mass as it appears in classical non relativistic case is the measure of material that is contained in the particle. That is one aspect. Mass is the one which appears the force acceleration equation. Larger the mass, lesser the acceleration for the same force. Mass is the one which decides gravitational attraction between two particles at a given distance.
Generally we take our definition of mass from Force-acceleration relations. This mass is also called Rest mass. Please note that this mass does not change as the particle gains speed. Very interestingly, material can be converted into energy. Once that happens material disappears and the mass is lost. The relation is E = mc^2.
People also define mass from this relation E = mc^2. The the energy of a particle is E, its "so called" mass is E/c^2. This is very different from the mass that we know from force-acceleration relations. The name given is relativistic mass. On fact we do not gain much from this definition and its use is gradually decreasing. You really dont need a new word. Energy serves the purpose and energy and relativistic mass differ by just a constant. I will use mass to mean the one that appears in dynamics equations which is same as Rest Mass. I even drop the adjective "Rest" as for me a particle has only one mass, whether it moves or not.
Photon has energy but no mass. If you still wish you can say it has relativistic mass E/c^2 but that has nothing useful. You are only adding a new word for energy.
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