Light is generally defined as the sense impression received by the eye. It was formerly believed that it was caused by streams of corpuscles emitted by the source of light. This was known as the emission theory. Early in the nineteenth century, the undulatory displaced the emission theory. According to this, light is a transverse vibratory motion extended longitudinally through the ether.
The experiments of Faraday, Maxwell, Fresnel, Hamilton, Green, and others suggested that the undulatory theory required for its validity a new medium different from the atmospheric air and from every substance known to man. Just as the results of investigations into reflection, refraction, diffraction, and polarization showed that the old corpuscular theory of light was untenable, so these experiments seemed to cast doubt upon both the undulatory and emission theories.
Fresnel, when studying problems in polarization, noticed that a theory of light proposed by Hooke appeared to be true. Hooke asserted that light vibrations are not longitudinal but transverse.
Fresnel found by his experiments that the idea of longitudinal vibrations acting along the line of propagation in the direction of the rays would not explain the polarization changes in light. They suggested that there was a transverse movement perpendicular to the ray. When Fresnel's researches were published, physicists realized that if the transverse direction of luminous vibrations was denied the undulatory movement of light would also be denied. Now transverse vibrations cannot exist in any medium resembling a fluid, because it is characteristic of fluids that, so long as the volume continues constant, its different parts can be displaced without the appearance of any reaction. This necessitates the assumption that light needs a solid body for its transmission and Lord Kelvin asserted that this body must be a solid more rigid than steel.
When the vibratory theory was accepted, it became necessary to investigate the nature of the ether and to determine its characteristic properties. Neumann, MacCullagh, Green, and Stokes then developed an elastic solid theory of the ether.
The experiments of Lord Rayleigh, Lorentz, Drude, Larmor, and others suggested that light is identical with electromagnetic disturbances and, consequently, is an electrical phenomenon.
Some of the finest developments in physics during the nineteenth century were in the realm of electricity. They resulted in an enormous extension of the use of electricity in industry and commerce and led to the investigation of radioactivities of various kinds and these in turn are developing investigations of a most brilliant character.