The ancient Greeks did not know what light is, what is its nature. However, they formulated laws that formed the basis of geometric optics. With the help of these laws, one can describe optical phenomena, use them to describe the processes occurring in various optical systems, create devices such as a telescope, microscope, and telescope. Then, almost simultaneously in the 17th century, two completely different theories about what light is and what its nature were, arose and began to develop. One of these theories is associated with the name of Newton (corpuscular theory), the other with the name of Huygens (wave theory).
According to the corpuscular theory, light is a stream of particles (corpuscles) emitted by luminous bodies. Newton believed that the movement of light corpuscles obeys the laws of mechanics. From the corpuscular theory it followed that the speed of light in media should be greater than the speed of light in vacuum.
The wave theory, in contrast to the corpuscular theory, considered light as waves propagating in a special medium – ether, which fills all space and penetrates into all bodies. According to the wave theory, the speed of light in media should be less than the speed of light in vacuum. The wave theory explained the laws of geometric optics, the phenomena of interference, diffraction, polarization of light. It was only in the middle of the 19th century that it was experimentally proved that the speed of light in matter is less than in vacuum. The wave theory has received universal acceptance. Only one thing confused the scientists. It was not possible to experimentally detect this hypothetical medium – ether
However, these difficulties were overcome. In the 60s of the 19th century, Maxwell established the general laws of the electromagnetic field, which led him to the conclusion that light is electromagnetic waves, the length of which is in the range from 0.4 µm to 0.78 µm. An important confirmation of this point of view was the coincidence of the speed of light in a vacuum with the speed of an electromagnetic wave. The electromagnetic nature of light was recognized after Hertz’s experiments on the study of electromagnetic waves (1887-1888).