Question

# The focal length of a bi-convex lens is 20 cm and its refractive index is 1.5. If the radii of curvatures of two surfaces of lens are in the ratio 1:2, then the value of the larger radius of curvature is: (in cm)

A
10
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B
15
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C
20
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D
30
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Solution

## The correct option is D 30From the data given in the question, μ=1.5 ; f=20 ; R1:R2=1:2 Let R1=R (say) then, R2=2R Using Lens maker's formula , 1f=(μ−1)(1R1−1R2) For a bi-convex lens, R1 is positive and R2 is negative. ∴1f=(μ−1)(1R1+1R2) Substituting the given data we get, 120=(1.5−1)(1R+12R) ⇒120=34R⇒R=15 cm ∴R2=2R=30 cm Hence, option (d) is the correct answer.

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