That part of Electromagnetic Radiation to which the human eye is sensitive. The Wavelengths of visible light range from c.400 to c.750 nanometers. If white light, which
contains all wavelengths, is separated into a Spectrum, each wavelength is seen to correspond to a different Color. The scientific study of the behavior of light is called Optics; it covers Reflection of light by a mirror or other object, Refraction of light by a lens or Prism, and Diffraction of light as it passes by an opaque object. Huygens, Christiaan proposed (1690) a theory that explained light as a Wave phenomenon. Newton, however, held (1704) that light is composed of tiny particles, or
corpuscules, emitted by luminous bodies. By combining his corpuscular theory with his
laws of mechanics, he was able to explain many optical phenomena. Newton's
corpuscular theory of light was favored over the wave theory until important
experiments, which could be interpreted only in terms of the wave theory, were done on
the diffraction and Interference of light by Young, Thomas (1801) and A.J. Fresnel (1814-15). In the 19th century the wave theory became
the dominant theory of the nature of light. The electromagnetic theory of Maxwell, James Clerk (1864) supported the view that visible light is a form of electromagnetic
radiation. With the acceptance of the electromagnetic theory of light, only two
general problems remained. It was assumed that a Massless medium, the ether, was the carrier of light waves, just as Air or Water carries Sound waves. The famous experiments (1881-87) by A. A. Michelson and E.W. Morley,
in which they tried unsuccessfully to measure the velocity of the earth with
respect to this medium, failed to support the ether hypothesis. With his special
theory of Relativity, Albert Einstein showed (1905) that the ether was unnecessary to the electromagnetic theory.
Also in 1905, Einstein, in order to explain the Photoelectric Effect, suggested that light, as well as other forms of electromagnetic radiation,
travel as tiny bundles of Energy, called light quanta (quanta is plural of quantum), or Photon(s), that behave as particles (see Quantum Theory). Light thus behaves as a wave, as in diffraction and interference phenomena,
or as a stream of particles, as in the photoelectric effect. The theory of
relativity predicts that the speed of light in a Vacuum (186,282 mi/sec = 299,792.458 km/sec) is the limiting velocity for material
particles.