According to Einstein's theory of special relativity, any particle that travels at the speed of light (c) must have zero rest mass. Photons are particles of light and are observed to travel at the speed of light in a vacuum.
The rest mass of a photon is
Asked in TNPSC CS Prelims 2015 · Topic: Wave Motion (Electromagnetic Waves: Properties, Electromagnetic Spectrum, Energy and Momentum)
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