# 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
1 year, 7 months 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.

- 1 year, 7 months ago

The 2nd is not. Obvio the first is

- 1 year, 7 months ago

I'm sorry, I don't understand what you're saying.

- 1 year, 7 months 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

- 1 year, 7 months ago

As I am saying mass should be defined, which is not only in photon's case

- 1 year, 7 months 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.

- 1 year, 7 months ago

If we change it according to refractive index, then only mass of photon will be undefined in all cases. Thats my point

- 1 year, 7 months ago