Optionals IAS Mains Physics: Questions 239 - 243 of 307

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Question 239

Appeared in Year: 2010

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Essay▾

What is meant by a dielectric? Define polarization vector and relate it with the average molecular dipole moment. Obtain expression for the potential due to a polarized dielectric in terms of the polarization vector.

Explanation

Dielectric are basically electric insulators which ordinarily do not contain any free charges carriers for conductors. They, however, contain positive and negative charges which are bound together and hence could be affected by the applied electric fields. The importance of insulators in affecting the electric fields was in fact realized quite earl…

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Question 240

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Appeared in Year: 2009

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Essay▾

What are the characteristics of stimulated emission? show that in the optical region, stimulated emission is negligible compared to spontaneous emission. (Paper I Marks - 10)

Explanation

Characteristics of stimulated emission:

  • The process of stimulated emission is controllable form outside.
  • The photon induced in this process propagates in the same direction as that of stimulating photon.
  • The induced photon has features identical to that of the inducing photon. It has the same frequency, phase and plane of polarisation as that of the s…

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Question 241

Interference of Light-Young's Experiment

Appeared in Year: 2009

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Essay▾

Let By using analytical method, obtain an expression to explain interference. Also show that intensity varies along the screen in accordance with the law of cosine square in interference pattern. (Paper I Marks - 10)

Explanation

  • As given
  • When is the phase difference between them. According to Young՚s principle of superposition, the resultant electric field at any point P due to the simultaneous action of the two wave is given by

Eqn (1) shows that the superposition of two sinusoidal waves having the same frequency but with a phase difference produces a sinusoidal wave with…

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Question 242

Principles of Fibre Optics Attenuation
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Appeared in Year: 2009

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Essay▾

Optical power of 1mw is launched into an optical fibre of length 100m. If the power emerging from the other end is 0.3mw, calculate the fibre attenuation. (Marks 7)

Explanation

If is the optical power launched at the input end of the fibre, then the power at a distance L down the fibre is given by

  • Where is the fibre attenuation coefficient expressed in units of
  • In units of , is defined through the equation

Question 243

Dulong-Petit, Einstein, and Debye's Theories of Specific Heat of Solids

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Essay▾

What are the limitations of Einstein՚s theory of specific heat of solids when compared with experiments at low temperature?

Outline the assumptions made in Debye՚s theory and show that the specific heat at low temperature follows law. What is the significance of Debye՚s temperature, ? (Marks 20)

Explanation

  • Although the Einstein՚s model provides a much better explanation for the variation of heat capacity with temperature than the Classical theory, it fails to account for the values of specific heat at very low temperatures. The Einstein՚s theory assumed that the atoms of a crystal vibrate totally independent of each other.
  • The vibrational motion of th…

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