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Question

A concave lens of glass , refractive index 1.5, has both surfaces of same radius of curvature R.On immersion in a medium of refractive index 1.75, it will behave as a :-

A
Convergent lens of focal length 3.5 R
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B
Convergent lens of focal length 3.0 R
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C
Divergent lens of focal length 3.5 R
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D
Divergent lens of focal length 3.0 R
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Solution

The correct option is A Convergent lens of focal length 3.5 R
Using lens maker formula :
1f=(nlensnsurr1)(1R11R2)
Given : R1=R R2=R nlens=1.5 nsurr=1.75
1f=(1.51.751)(1R1R)
f=+3.5R
Since the focal length of the lens immersed in medium is positive, thus the concave lens behaves as a convergent lens in that medium.

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