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One of my friend told me that photon is massless but I think that (please correct me if i was wrong), Albert Einstein's equation $E=mc^2$.

This equation stands for energy, now as we know energy is also, $E= hc/\lambda$ here h is planck constant and c is speed of light and $\lambda$ is wavelenght of upcoming photons wavelenght so from both equation $mc^2 =hc/\lambda$ solving further $m=h/c\lambda$ hence the photon is not massless.

Tell me where I am wrong.

Curious one
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  • This question should certainly be closed, but at least some of the alleged duplicates are not duplicates. They ask "Does a photon have mass?". This one asks "Why doesn't Einstein's equation imply that a photon has mass?". That's off topic for multiple reasons, but a very different question nonetheless. – WillO Aug 24 '16 at 17:03
  • @WillO Not sure I agree. The answer is "Using $E = mc^2$ as a definition of mass is incorrect, the right definition is...". – dmckee --- ex-moderator kitten Aug 24 '16 at 17:37

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The actual relation is $E^2$ = $(pc)^2$ + $(mc^2)^2$ where m is the rest mass of the particle. Photons do not posses rest mass making m zero. Thus $E$ = $pc$ for photons.

Lelouch
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