Q.

Two Plano concave lenses of glass of refractive index 1.5 have radii of curvature  20 cm   and 30 cm  respectively.  They are placed in contact with the curved surface towards each other and the space between them is filled with a liquid of refractive  index 5/2. The focal length of the combination is (in cm)

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a

– 92

b

108

c

6

d

12

answer is D.

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Detailed Solution

From lens maker’s formula, effective focal length is

1F=(μ1μg)(1R1+1R2)

      =(5232)(120+130)

1F112F=12cm

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