The theory of relativity having new concept of space and time, which is applicable not only to mechanical phenomena, but also to all optical and electromagnetic phenomena, which is divided into two parts

1. Special or restricted theory of relativity
2. General theory of relativity

The special theory of relativity leads with systems known as inertial systems, that is, the systems which move in uniform rectilinear motion relative to one another. According to this “All systems of co-ordinates are equally suitable for description of physical phenomenon”. If we extend this principle of accelerated systems that is the systems moving with acceleration relative to one another the theory of relativity is known as General Theory of Relativity. The General Theory of Relativity is applicable to the laws of gravitation and explains it in a more refined manner than given by Newton.

Postulates of special theory of relativity

1. The fundamental laws of physics have the same form of all inertial systems (that is for all reference system at rest or moving with constant linear velocity to one another).
2. The velocity of light in vacuum is independent of the relative motion of the source and the observer.

These are two fundamental postulates are used in the special theory of relativity. The first postulate is the extension of the conclusion drawn from Newtonian mechanics; since velocity is not absolute, but relative, which is a fact drawn from the failure of the experiments to determine the velocity of Earth relative to ether. We know that the speed of light is not constant under Galliean transformations and the first postulate is the conclusion from Newtonian mechanics; the second postulate is not true according to Galliean transformations. Actually, this is true since the velocity of light calculated by any mean is constant. Thus, the second postulate is very important and only this postulate is responsible to differentiate the classical theory of Einstein’s theory of relativity and according to Einstein, the theory of relativity is applicable to law of optics.