KVPY SX SB Exam 20141 Question Paper

Question : 31 of 120

Marks: +1, -0

The flat face of a plano-convex lens of focal length 10 cm is silvered. A point source placed 30 cm in front of the curved surface will produce a

Solution:

f = 10 cm
After silvering of flat face lens behave as mirror of focal length f

_\eq

1/\f_\eq=1/\f_1+1/\f_2+1/\f_3

1/\f_\eq=2/\f_1+1/\f_2

1/\f_\eq=2/10+1/∞

\f_\eq = 5

mirror formula

1/\f=1/u+1/v

1/{-5}=1/{-30}+1/v

v = – 6 cm
Image is real and 6 cm away from silvered lens.

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