Question

Focal length of convex lens is 20 cm and its ri is 1.5

Answer 1

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The radius of curvature of a plano - convex . lens is 20 cm If the refractive index of the material of the lens be 1.5 it will

act as a convex lens only for the objects that lie on its curved sideact as a concave lens for the objects that lie on its curved sideact as a convex lens irrespective of the side on which the object lies act as a convex lens irrespective of side on which the object lies

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A)

Solution :

Given μ=1.5

If the object lies on the plane side,

then R1=∞ and R2=−20cm

1f=(μ−1)[1R1−1R2]

=(1.5−1)[1∞+120]

=140

f=40cm

Hence it behave as a convex lens If the object on its curved side.

If the object on its curved side .

R1=20cm

R2=∞

1f1=(μ−1)(1R1−1R1)

=(1.5−1)(120−1∞)

=140

f′=40cm