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Question

Find the focal length of the lens shown in figure. The radius of curvature is R=10 cm and the refractive index of material of the lens is 32.


A
15 cm
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B
+20 cm
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C
+10 cm
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D
30 cm
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Solution

The correct option is C +10 cm
Lens maker's formula for convex lens if object is at infinity,
1f=(μ2μ11)(1R11R2)
Here,
μ2=32, μ1=1, R1=+10 cm and R2=10 cm


1f=(321)(110110)

or, 1f=12(210)

f=+10 cm
Why this question?
Tip: In such questions, be very careful with the sign convention of R1 and R2.
It is recommended to select positive in the direction of incidence.

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