Page 91 - Physics - XII
P. 91

For normal view, D will be near B.

                                              BC
          Therefore,                    µ  =
                                       a w   BC'
          If small quantity of water in concave mirror B will be very near to P, i.e.
                                       BC ∼ PC and BC' ∼ PC'
                                              PC       Real radius of curvature of mirror
          ∴                             µ  =      =
                                       a w   PC'    Apparent radius of  curvature of mirror

          Procedure
          1.  Place the concave mirror on a horizontal surface (plane), so that its principal axis is along vertical.
          2.  Hold the optical needle horizontally in a clamp stand, so that its tip lies just above the pole P and at a
              distance equal to 2f (f is focal length) as shown in Fig. 8.1.
          3.  Remove the parallax between the needle and its image.

          4.  Mark the real and inverted image of the optical needle in the mirror. Note the reading of this image.
          5.  Measure the distance (PC) using plumb line and metre scale.
          6.  This measured distance is the actual radius of curvature of the concave mirror.
          7.  Now, add small amount of water to the concave mirror which will change the position of image needle.

          8.  Adjust position of the optical needle so that its position shift from C to C' to remove parallax between
              the needle and its shifted image is seen.
          9.  Measure the distance (PC') which is the apparent radius of curvature of the concave mirror.
          10. Repeatedly do the experiment for four to fi ve times and record the readings.


          Observations
          Rough focal length of the concave mirror, f = _____ cm

                                                 Table for refractive index

             S.       Actual radius of         Apparent radius of                  R                 Mean µ
            No.     curvature (R) in cm       curvature (R') in cm                 R'

             1.

             2.

             3.

             4.


          Calculations

                                                                                       R
          Putting the observed values of actual radius R and apparent radius R' in       , and calculate the value of
          refractive index of the given medium.                                           R

          Result
          Refractive index of water, µ = _____



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