III.

Theory

The British physicist James Clerk Maxwell laid out the theory of electromagnetism in a series of papers published in the 1860s. He deduced that electromagnetic waves must exist and stated that visible light consisted of such waves.

Physicists had known since the early 19th century that light travels as a transverse wave (a wave in which the vibrations move in a direction perpendicular to the direction of the advancing wave front). They assumed, however, that the wave required some material medium for its transmission, so they thought that there was an extremely diffuse substance, called ether, which was the unobserved medium. Maxwell's theory made such an assumption unnecessary, but the ether concept was not abandoned immediately, because it fitted in with the Newtonian concept of an absolute space-time frame for the universe. A famous experiment conducted by the American physicist Albert Abraham Michelson and the American chemist Edward Williams Morley in the late 19th century undermined the ether concept and was important in the development of the theory of relativity. This work led to the realization that the speed of electromagnetic radiation in a vacuum is the same, regardless of the velocity of the source or the observer.