[align=left]بسم الله الرحمن الرحيم
Photons are traditionally said to be massless This is a figure of speech that physicists use to describe something about how a photon's particle-like properties are described by the language of special relativity.
In special relativity, it turns out that we are still able to define a particle's momentum p such that it behaves in well-defined ways that are an extension of the newtonian case
[align=center]P = m*v[/align]
When the particle is at rest, its relativistic mass has a minimum value called the "rest mass".
It also turns out that in special relativity, we are able to define the concept of "energy" E, such that E has simple and well-defined properties just like those it has in newtonian mechanics. When a particle has been accelerated so that it has some momentum p (the length of the vector p) and relativistic mass mrel, then its energy E turns out to be given by
[align=center] E = mrelc2 , and also E2 = p2c2 + m2restc4 . .....................(1)[/align]
In classical electromagnetic theory, light turns out to have energy E and momentum p, and these happen to be related by E = pc. Quantum mechanics introduces the idea that light can be viewed as a collection of "particles": photons. Even though these photons cannot be brought to rest, and so the idea of rest mass doesn't really apply to them, we can certainly bring these "particles" of light into the fold of equation (1) by just considering them to have no rest mass. That way, equation (1) gives the correct expression for light, E = pc, and no harm has been done. Equation (1) is now able to be applied to particles of matter and "particles" of light.[/align]
[align=center]:smile_71:[/align]