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PHE 9

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              BACHELOR OF SCIENCE (B.Sc.)
00
v—i                   Term-Fld Examination

                              ) Inc.,. 2011
CD
                              PHYSICS
                           PIT-.{) : OPTICS

  Time : 2 hours                               Maximum Marks : 50
  Note : All questions aw compulsory hat there are internal
             choices. The fOr each question are indicated
             against it. You cm' use lu tables or a calculator. Symbols
             have their usual meimings.

  1.   Attempt any five pdris :                               2x5 =10

       (a)      For a double slit interference experiment,
                what is the nature/shape of fringes
                obtained on the screen ? Plot the intensity
                distribution us a function of phase
                difference.

       (b)      The energy ncated by the sun per second
                is 4 x 1026 Js It the sun is a sphere of
                radius 7x 10' km, calculate the Poynting
                vector at its s-, li face,

       (c)      When sunlight is R;ident on the surface of
                a calm pond water at an angle of 3T, the
                reflected light is completely polarised.
                Justify.


  PHE-9                             1                           P,1 .0.
     (d)   The resolving power of a Fabry - Perot
           interferometer for normal incidence is
           5.42 x105. Calculate the separation
           between the wavelengths that can be just
           resolved at X=500
     (e)   List two important characteristics of single
           slit Fraunhofer diffraction pattern.
     (f)   To penetrate deeper in space, we use
           telescopes with objectives of larger aperture.
           Justify this statements on physical basis.
     (g)   light of wavelength 400 rim has a wavetrain
           30 X long. Calculate coherence time.
     (h)   Draw energy level diagram of Nd ion in
           Nd : YAG laser.


2.   Answer any two parts :                            5x2=10
     (a) Consider two light waves of unequal 3+2
          amplitudes and having an arbitrary phase
          difference, moving along the Z-direction.
          Their electric field vectors are mutually
          orthogonal. Show that (i) their superposition
          leads to an elliptically polarised light and
          (ii) linear polarisation and circular
          polarisation states are special cases of elliptic
          polarisation.
     (11) Starting from Maxwell's field equations in        5
          free space, show that light waves are
          electromagnetic in nature.
     (c)  (i)   Interference pattern is obtained by 3+2
                division of wavefront as well as
                division of amplitude. Explain.
          (ii) List one instrument used to observe
                each type of pattern.


PHE-9                         2
3.   Answer any one part :                         10x1=10
     (a) (i)   Explain the formation of Newton's 4+3
               rings and discuss how these can be
               used to determine wavelength of light.
         (ii) Tn a Newton's rings experiment, the        3
               diameters of the 5th and 15th rings are
               0.336 cm and 0.590 cm, respectively.
               Calculate the radius of curvature of
               the piano-convex lens if light of
                 wavelength 5890 A is used.
     (b)   Explain the principle of Fabry-Perot
           interferometei. Derive an expression for the
           intensity of transmitted light and discuss
           sharpness of fringes. Flow is it superior to
           Michelson interferometer ?                  3+5+2
4.   Answer any one part :                        10x1=10
               Discuss spatial evolution of Fresnel
              diffraction pattern to Fraunhofer
              diffraction.                          3+5+2
              Show that a zone plate acts like a
              multi-foci converging lens.
              A zone plate is illuminated by light of
                wavelength 5500 A . If radii of zone
                plate are expressed in cm and given
                                    -
                by the relation ru =0.1 N/i, calculate
                the location of first focal point.
                Discuss Rayleigh's criterion for
                resolving two images.                3+4+3
                Obtain an expression for resolving
                power of a microscope.
                Explain why we have to use an
                electron microscope to study finer
                details of crystal structures.


PHE-9                        3                       P.T.O.
5.   Answer any one part :                           10)(1=10
     (a)   Derive expressions for Einstein's A and B 5+5
           coefficients. Explain how Finsiein's
           prediction of population inversion was
           realised in actual practice by Maiman.
     (b)   (i)   Holography is better than photography. 5
                 Discuss on the basis of the processes
                 involved.
           (ii)   Discuss salient features of a hologram. 24.3
                  With the help of a schematic diagram,
                  explain the process of image
                  reconstruction.




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PHE.-9                                               7                                               P.T.O.
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PHE-9                                    8