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Question

A convex lens is made of a glass of refractive index 1.5. The radius of curvature of each of its surface is 20cm. Then the ratio of the power of the lens when placed in air to its power when immersed inside a liquid of refractive index 1.25 is

A
2:5
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B
5:2
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C
3:2
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D
2:3
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Solution

The correct option is B 5:2
Given - ang=1.5,anl=1.25
By using , the focal length of a glass (g) lens in a medium (m) is ,
1f=(mng1)[1/R11/R2]
When lens is in air ,
1fa=(ang1)[1/R11/R2] .............eq1
When lens is in liquid ,
1fl=(lng1)[1/R11/R2] ................eq2
now lng=ang/anl=1.5/1.25
dividing eq1 by eq2 ,
flfa=(ang1)(lng1)
or flfa=(1.51)(1.5/1.251)=5/2
or PaPl=52 (because f=1/P)
Hence Pa:Pl=5:2

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