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Question

A convex lens of focal length 15 cm is kept in front of a concave lens of focal length 30 cm. What is the power of the two lens system?


  1. - 0.033 D

  2. + 0.033 D

  3. - 0.6 D

  4. + 0.6 D


Solution

The correct option is B

+ 0.033 D


Let P be the power of lens, 
f be the focal length of lens
f1 - focal length of a convex lens, P1 is the power corresponding to this lens
f2 - focal length of a concave lens, P2 is the power corresponding to this lens
Ptotal - Power of combination of lens 
As per the sign conventions,
f1=+15 cm
f1=30 cm
(Ptotal) = P1P2

             = 1f11f2   [As P=1f (in m)]
             = 10.1510.30
             = 210.30
             = 10.30
Ptotal = + 3.33 D

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