NEET Physics Ray Optics and Optical Instruments Questions Solved

BOARD

Use the mirror equation to show that an object placed between 

f and 2f of a concave mirror produces a real image beyond 2f.     (2 marks)

1f=1v+1u                                                              (1/2 mark)

For concave mirror f<0 and u<0

As object lies between f and 2f

(i) At u=-f

1v=-1f+12f=-12f                                        (1/2 mark)

=-2f                                                                 (1/2 mark)

Hence, image distance v-2f                             (1/2 mark)

Since v is negative, therefore, the image is real.

Alternative Method

1f=1v+1u                                               (1/2 mark)

For concave mirror                                      (1/2 mark)

f<0, u<0

so, 2f<u<f

12f>1u>1f12f-1f>1u-1f>1f-1f12f>1v>0   so, 1u>1f>1-v

12f<1v<0                                                       (1/2 mark)

v<0    so, Image is real

Also v>2f        image is formed beyond 2f.                 (1/2 mark)

(Any alternative correct method should be given full credit.)

Difficulty Level:

  • 82%
  • 19%
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