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Question

An equi-concave lens is to be made of crown glass. What should be radii of the surfaces of the lens if the power of the lens is −2.5 dioptre and refractive index of crown glass is 1.65?

A
52m
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B
62cm
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C
52cm
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D
520cm
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Solution

The correct option is C 52cm
Given power of lens =2.5D Power of lens =100f( cm) where f= focal length of the lens so 2.5=100ff=10025f=40 cm
Refractive index of lens =1.65
equi-concave lens having radius =R
1f=(μ1)(1R11R2)
140=(1.65)(1R1R)
140=(0.65)2RR=40(0.65)2.R=52 cm
so option c is correct

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