Question

what is Photon? And prove that the rest mass of photon is zero.

Answer 1

But according to special relativity, light ALWAYS travels with the light speed c, and is NEVER at rest. And so it has zero REST mass. which means that though photons don't have rest mass, they do have energy and thus they have mass. The photons are wave particles.

Let's just use the two postulates of special relativity. From this, we have

       m=m01−v2c2√

Here, m is the total energy of the particle (Einstein's relation makes sense in this respect) and m0 is the rest mass of the particle.

Suppose, for instance, we didn't know the rest mass of the photon. However, we knew it traveled at the speed of light, c. Then, we find:

       limv→cm→m00→{Eundefinedm0→0otherwise

Since we clearly have an idea that photons have energy, given the photoelectric effect (also Einstein), it makes more sense to mathematically (and physically) define the rest mass of the photon to be zero. This is one of the reasons why "nothing with rest mass can travel at the speed of light".

Another addition: a photon has no rest frame. There's two ways to think about that. (1) No matter what you do, you'll never find an inertial frame in which the photon is at rest. (2) You'll never catch up to a photon, ever. Why does this make sense? A postulate of SR is that the speed of light is the same in all inertial frames. This means that in the inertial, rest frame of the photon -- it travels at the speed of light while also being simultaneously at rest. This is a paradox, therefore it has no rest frame. Since it has no rest frame, we can't define a finite rest mass for it.

Answer 2

Hey mate,

Here is your answer :

The photon is a type of elementary particle. It is the quantum of the electromagnetic field including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always move at the speed of light in vacuum, 299792458 m/s.

Spin: 1

C parity: −1

Parity: −1

Symbol: γ

Mean lifetime: Stable

Statistic: Bose–Einstein statistics

Interactions: Electromagnetic, Weak, Gravity

Hope it helps.