question_answer

Answer the following questions : (a) When monochromatic light is incident on a surface separating two media, the reflected and refracted light both have the same frequency as the incident frequency. Explain why.                                                                 (b) When light travels from a rarer to a denser medium, it loses some speed. Does the reduction in speed imply a reduction in the energy carried by the light wave? (c) A narrow pulse of light is sent through a medium. Will you expect the pulse to retain its shape as it travels through the medium? (d) In the wave picture of light, intensity of light is determined by the square of the amplitude of the wave. What determines the intensity of light in the photon picture of light? (e) The speed of light in still water is \[c/\mu \]where \[\mu \], is the refractive index of the water. What is the speed of light in a stream of water flowing at a steady speed of \[\upsilon \]relative to the observer?                                 

Answer:

                (a) Both reflection and refraction occur due to interaction of light with the atoms at the surface of separation. These atoms may be regarded as oscillators. Light incident on such atoms forces them to vibrate with the frequency of light. As the light emitted by these charged oscillators is equal to their own frequency of oscillation, so both the reflected and refracted lights have the same frequency as the frequency of incident light. (b) No, the reduction in the speed of light does not imply the reduction in the energy of the light wave because the energy carried by a wave depends on the amplitude of the wave, not on the speed of wave propagation. (c) No. A pulse can be viewed as being made of harmonic waves with a large range of wavelengths. Since the speed of propagation in a medium depends on wavelength, different wavelength components of the pulse travel with different speeds. Hence according to Huygen's theory of wave propagation, the pulse will not retain its shape as it travels through the medium. (d) In the photon picture of light, intensity of light at a point is determined by the number of photons incident per unit area around that point. (e) The speed of light in water depends on the relative motion between the observer and water. One may expect the answer to be\[\frac{c}{\mu }+\upsilon \]. According to the special theory of relativity, the correct speed of light in the stream of water is \[c'=\frac{c}{\mu }+\upsilon \left( 1-\frac{1}{{{\mu }^{2}}} \right)\],for \[\upsilon <