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Thermodynamics

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Nature of Light

(i) Corpuscular theory: Rectilinear propagation of light is a natural deduction on the basis of corpuscular theory. This theory can also explain reflection and refraction, through the theory does not clearly envisage why, how and when the form of attraction or repulsion is experienced perpendicular to the reflecting or refracting surface by a corpuscle. Newton assumed that the corpuscles possess fits which allow them easy reflection at one stage and easy transmission at the other. According to Newton’s corpuscular theory the velocity of light in a denser medium is higher than the velocity in a rarer medium. But the experimental results of Foucault and Michelson show that the velocity of light in a rarer medium is higher than that in a denser medium. Interference could not be explained on the basis of corpuscular theory because two material particles cannot cancel one another’s effect. The phenomenon of diffraction viz. bending of light round corners or illumination of geometrical shadow cannot be conceived according to corpuscular theory, because a corpuscle travelling at high speed will not be deviated from the straight line path. Certain crystals like quartz, calcite etc exhibit the phenomenon of double refraction. Explanation of this has not been possible with the corpuscle concept. The unsymmetrical behaviour of light about the axis of propagation (viz. polarization of light) cannot be accounted for by the corpuscular theory.

(ii) Wave theory: Huygens wave theory could explain satisfactorily phenomena of reflection and refraction. Applying the principle of secondary wave points, rectilinear propagation of light can be correlated. The phenomenon of interference can also be understood considering that light energy is propagated in the form of waves. Two wave trains of equal frequency and amplitude and differing in phase can annul one another’s effect and produce darkness. Similar to sound waves, bending of waves round obstacles is possible, thus enabling the understanding of the phenomenon of diffraction. Double refraction can also be explained on the basis of wave theory. According to Huygens, propagation of light is in the form of longitudinal waves. But in the case of longitudinal waves, one cannot expect the unsymmetrical behaviour of a beam of light about the axis of propagation. This difficulty was overcome when Fresnel suggested that the light waves are transverse and not longitudinal. On the basis of this concept, the phenomenon of polarization can also be understood. Finally, on the basis of wave theory it can be shown mathematically, that the velocity of light in a rarer medium is higher than the velocity of light in a denser medium. This is in accordance with the experimental results on the velocity of light.

(iii) Conclusion: The controversy between the corpuscular theory and the wave theory existed till about the end of the eighteenth century. At one time the corpuscular theory held the ground and at another time the wave theory was accepted, the discovery of the phenomenon of interference by Thomas Young in 1800 the experimental results of Foucault and Michelson on the velocity on light in different media and the revolutionary hypothesis of Fresnel in 1816 that the vibration of the either particles is transverse and not longitudinal wave, in a way, a solid ground to the wave theory.

The next importance advance in the nature of light was due to the work of Clerk Maxwell. Maxwell’s electromagnetic theory of light lends support to Huygens wave theory whereas quantum theory strengthens the particle concept. It is very interesting to note, that light is regarded as a wave motion at one time and as a particle phenomenon at another time.