Why the speed of light is constant i.e.

c = 3*10^8 m/s

can it be slower or higher than this.

The nasty answer:

If you are talking about the value of c, then it is constant because we defined it to equal some constant. From the "legal" point of view, c is constant and only the length of one meter could change. That's due to some unintuitive definition in the SI-units and probably not what you wanted to know.

 

The real answer:

What you probably meant is if light-pulses (in vacuum) could travel at a different speed than c. Yes, that is possible ("every thing is possible as long as you can imagine it, that's what being a magical elf is all about" - Cubert Farnsworth) but would violate some very successful physics model (electrodynamics). So since there's no (generally accepted) indication that light-pulses didn't travel at c, keeping a very successful model seems preferable (to me).

We've been told from Prof.Rafiq that the speed of light is constant only in vacuum and vacuum does not exict. Hence, it is not constant.

And the proof was that light is bendable.

 

My question is if light is really bendable, how would this fact proof that speed of light is not constant?

 

And is photons really massless?

My question is if light is really bendable, ...

As always: Depends on what that means. But since we usually say light-rays are bent due to gravity, I suppose the answer is yes.

... how would this fact proof that speed of light is not constant?

My guess is "not at all". The calculations for the bending are done assuming the speed to be constant.

 

And are photons really massless?

See post #2. Massless is the best guess we have and compatible with experimental data.

 

Please use the default fontsize as default fontsize.

"speed of light" can be used two ways. One is a constant, c, which is the speed light would have in a vacuum. The speed that light actually has, in a given medium, will be c/n, where n is the index of refraction of the medium.

 

Usually when one uses the phrase it is accepted that we are talking about the constant.