Plz explain the concept of electromagnetic waves.

The speed of electromagnetic radiation in free space-which we shall call the speed of light-is central not only to Maxwell's theory of  Electromagnetism but also to Einestein's theory of relativity. Its value has been measured so often and so precisely that in a sense, we can say that it has been " measured to death ." We have in mind the fact that the speed of light has been assigned an exact value by definationas part of the 1983 redefinition of the meter, namely,

c= 299,792,458 m/s (exactly)

Thus the long history of measuring the speed of light is over. Today, if you send a light beam from one point to another (in fre space) and measure its transit time, you are not measuring the speed of light: you are measuring the distance between the two points.

When we say that the speed of electromagnetic waves in free space is 299,792,458 m/s, what reference frame are we talking about. It cannot be the medium through which the light waves travels because,in contrast to sound, no medium is required.

Physicists of the 19th century, influenced as they then were by an analogy between light waves and sound waves or other purely mechanical disturbances, did not accept the idea of a wave requiring no medium. They postulated the existence of an ether, which was a tenuous substance that filled all space as a medium of transmission for light.

Although it proved useful for many years, the ether concept did not survive the test of experiment.

In particular, careful attempts to measure the speed of the earth through the ether always gave the result of zero. Physicists were not willing to believe that the earth was permanently at rest in the ether and that all other bodies in the universe were in motion through it.

In 1905 Einstein resolved the delemma by making a bold postulate that we have already stated in section 17-7: if a number of observers are moving (at uniform velocity) with respect to each other and to a light source and if each observer measures the speed of a light emerging from the source, they will all measure the same vale.

"The speed of light in free space has the same value c in all directions and in all inertial reference frames."