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# Mass of photon and Einstein's theory of relativity

$$m=\frac {m_{0}}{\sqrt {1-\frac {v^{2}}{c^{2}}}}$$

As speed of light changes in a medium, should we change the denominator also?

If we dont change the denominator, then for a photon travelling with a high fraction of speed of light in a medium (less than c), mass should be defined, but it is not.

Note by Prince Loomba
7 months, 3 weeks ago

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The speed of the photon is same regardless of the medium.

It is it's apparent speed that we observe (which is lower due to collisions with the particles of the medium).

Note: This argument holds for any mass less particle as well. · 7 months, 3 weeks ago

The 2nd is not. Obvio the first is · 7 months, 3 weeks ago

I'm sorry, I don't understand what you're saying. · 7 months, 3 weeks ago

I am saying the first point is valid for all, for all particles it should be such. But the second point is for photon specially · 7 months, 3 weeks ago

As I am saying mass should be defined, which is not only in photon's case · 7 months, 3 weeks ago

Mass of what? The rest mass of the photon is zero. And the above formula is consistent with the fact that the moving mass of a photon is not defined. · 7 months, 3 weeks ago