Thursday, January 05, 2006

History of Optics

The history of Optics is made interesting by the frequent swings of physicists' viewpoints about whether light is of a wave nature or of a particle nature. The investigations about the nature of light have made significant contributions to the development of an altogether different branch of physics known as quantum physics, one of the crowning achievements of the physicists of the 20th century.

Isaac Newton, the great physicist, wrote the classic Optiks in which he has given an account of the numerous experiments he performed to study light. He believed light to be a stream of particles or corpuscles. The corpuscular model of light was originally proposed by Descartes. The corpuscular model of light was in tune with Netwon's laws of motion, he believed that these corpusles travel in straight lines when not acted upon by external sources. This explained the rectilinear propagation (i.e. travelling in a straight line) of light. A particle incident upon a plane smooth surface can be shown to have equal values for the angles of incidence and reflection. Thus, the well-known laws of reflection for light on a plane surface could be accounted for by the corpuscle picture. He tried to explain the laws of refraction by assuming that particles of a denser medium attract the corpuscles of light hence causing it to bend at the surface.

Christian Huygens (1629-1695), a Dutch physicist and a contemorary of Newton belived that light was a wave and with the so called "Huygen's Principle", explained the laws of reflection and refraction.

Although the wave picture of light wasn't taken too seriously initially, but in 1801, Thomas Young performed the famous double-slit interference experiment and gave a theoretical explanation using the wave theory. In 1814, Fresnel performed an experiment showing diffraction of light and explained it using the wave picture. He also explained the rectilinear propagation of light using the wave theory.

The wave theory was finally widely accepted after Foucault showed that the speed of light is less in water than in air, while Newton's particle-picture suggested that the speed of light should increase in a medium for refraction to take place.

Further, in the year 1857, Maxwell gave the famous Maxwell's equations of electromagnetism. From these laws, he could derive a wave equation. In such waves, a changing electric field creates a magnetic field and the changing magnetic field, in turn, creates an electric field, and the wave propogates. He called these waves electromagnetic waves. He also derived the speed of such a wave and found that this speed, in vacuum, matched the speed of light known at that time by Kohlrausch and Weber.

With making a statement about "faith in rationality of nature", he gave the electromagnetic theory of light according to which lights is an electromagnetic wave. This was experimentally verified by Heinrich Hertz in 1888.

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