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Sunday, April 8, 2012

Nature and source of light


Light is a form of energy which on entering our eyes gives the sensation of sight, and enables us to see various objects. Since the early times, scientists have made discussion on the true nature of light and have postulated various theories. These are discussed below:
A.      Newton’s corpuscular theory: Newton, in 1678, put forward this theory that light consists of tiny particles, called corpuscles, which are shot out by luminous objects such as an electric lamp, candle, etc. the corpuscles are tiny mass less particles, shot out at a speed of about 3*10⁸ ms⁻¹ from the lighted object. According to newton, these particles travel in straight lines and bounce off or pass into an object on striking it. On entering the eyes, these corpuscles cause the sensation of vision and the various colors are due to different sizes of the corpuscles. The phenomenon of refraction was explained by starting that these corpuscles are attracted by the material of the denser medium. Thus, the speed of light is increased in the denser medium and this resulted in a change in the direction. This theory is based on the fact that the speed of light must be greater in the denser medium. However, it was found later that light travels slowly in a denser medium than in rarer medium. This theory could not explain interference, diffraction and polarization of light also. Therefore, the corpuscular theory was discarded as an incomplete and inaccurate theory.
B.      Huygens wave theory: christian Huygens, a Dutch physicist, in 1690 proposed a new theory about the nature of light. According to this theory, light propagates from the source in the form of a wave. For the propagation of wave a medium is necessary. So, it was assumed that all space including vacuum is filled with medium called ether, which had the property of both elasticity and inertia. From the source of light, periodic disturbances are produced, which travel in the form of waves and thus energy is equally distributed in all directions. Huygens assumed that light waves are longitudinal. But Fresnel, using the phenomenon of polarization, proved that light waves are transverse in nature. Using wave theory, Huygens could explain reflection and refraction. Young explained interference and Fresnel explained diffraction and polarization on the basis of the wave theory.
C.      Electromagnetic theory: Maxwell in 1905 proved theoretically that light gets propagated in the form of electromagnetic waves, consisting of electric and magnetic fields mutually perpendicular as well as transverse to the direction of propagation of light. The electromagnetic waves propagate in free space with the velocity of light. Hertz demonstrated experimentally that electromagnetic propagate waves with the velocity equal to that of light.



D.      Quantum theory: Einstein in 1905 proposed a new theory of light, called a quantum theory. In order to explain the photoelectric effect. According to this theory, light is transmitted as tiny packets of energy called photons and energy of each photon is given by, E=hf, where f is the frequency of light and h is the plank’s constant.






E.       Duel nature of light: From the discussion of the various theories of light, we conclude that light can exist in particle form as well as wave form. Using quantum theory,we can explain photoelectric effect.But this theory could not explain interference, diffraction and polarization. These phenomena can be explained using wave theory. but wave theory could not explain photoelectric effect. so we have to accept the dual form of light viz. particle form and wave form.

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