A history of the study of light
Many of the early scientists who studied sound also studied light. Pythagoras, for example, believed that light came from visible objects toward the eye. However, in addition to this basically correct thought, Plato and many other Greeks, also held the mistaken belief that vision issued out from the eye. Despite this, many of the ideas of the ancient Greeks were accurate. The philosopher Empedocles correctly believed that light traveled with finite speed. Aristotle, too, conjectured about light as well as sound. He rightly explained rainbows as a sort of reflection off of raindrops. The mathematician Euclid worked with mirrors and reflection, and many other thinkers observed refraction, though they did not know how to express it mathematically. Ptolemy is the first recorded person to experiment with optics and collect data, but he believed in Plato's mistaken thought that vision extended out from the eye. Ptolemy's work was further developed by the Egyptian scientist Ibn al Haythen, who was known to Europeans as Alhazen. It was Alhazen who first drew ray diagrams, and who managed to discount Plato's theory, through a mixture of logic and experimentation. His work, done during the time of the Moorish Empire, was influential in later studies of light.
Many advances in the study of light were made in the 16th and 17th centuries by such renowned scientists as Galileo Galilei, Johannes Kepler, and Renes Descartes. Both Descartes and Dutch mathematician Willebrord Snell independently developed the law of refraction, basing much of their work on the earlier work of Alhazen. Snell discovered it earlier, however, and the law is now known as Snell's law.
During the late 17th century, a debate grew over whether light behaved as a particle or a wave. Sir Isaac Newton believed in a particle theory of light, in part because he did not observe diffraction in light, a property it should have had, had it been a wave. Though he did not know it, this was due a lack of coherency in the light when he was experimenting. Coherency is important, because without a steady, coherent beam, it is impossible to observe such things as diffraction of light. These days, coherency is generally achieved with a laser, a modern invention that was unavailable to Newton. There were those among his contemporaries, however, who believed in a wave theory of light. The most notable among these was the Dutch scientist Christiaan Huygens, who first wrote of light as a wave. Though Newton had difficulty explaining certain phenomena of light, it seemed that the wave theory also had difficulty explaining certain optical phenomena. Because of Newton's prestige, the particle theory was accepted for almost a century.
It was not until the early 19th century that the wave theory of light became widely accepted. This acceptance came in large part due to the work of the English doctor Thomas Young. In 1801 he reported his famous double-slit experiment, which clearly showed light to diffract, and thus travel as a wave. By passing light through a single slit before passing it through a double slit, he managed to emulate a point source of light, and to achieve the coherency that had eluded Newton. In the 1850s Fizeau and Foucault showed through measurements that light traveled slower through denser media. In the same century, Augustin Fresnel and later James Clerk Maxwell, working within a wave theory of light seemed to explain phenomena that Newton had been unable to explain in terms of a particle theory of light, such as polarization, interference, and diffraction. They were also able to address the question of what determines which part of the light will be reflected and which transmitted when light is reflected at a surface such as glass or water. This question was opened back up in the 20th century with the rise of quantum theory.
It was the Irish mathematician Sir William Hamilton who developed a theory that joined optics and mechanics, thus elucidating the relationship between wave and particle viewpoints. His theory helped lay the foundation for the later development of quantum mechanics
James Clerk Maxwell did much more to aid the study of light. In essence, he was the father of our modern perception of light. Working from Michael Faraday's discoveries in electricity and magnetism, he managed to fully explain electromagnetic phenomena, developing a theory that described how electromagnetic waves traveled through space. His theory was later experimentally proven correct by Heinrich Rudolph Hertz, the man whose name gives us the unit hertz, one of which is equivalent to one cycle per second. The velocity of the electromagnetic waves that Maxwell had described was found to be the same as that of light, essentially proving that light was an electromagnetic wave.
Maxwell's work seemed to have finally settled the issue of whether light was a wave or a particle, but the whole debate was reopened in the 20th century. Scientists such as Albert Einstein, who described the Doppler effect for light, brought the particle theory back into the picture with quantum theory. This time they postulated that light did not just behave as a particle or a wave, but had properties of both.
