Question

Diameter of plano-convex lens is 6 cm and thickness at the centre is 3 mm. If speed of light in material of lens is , the focal length of the lens is:

A

20 cm

B

30 cm

C

10 cm

D

15 cm

Hard
JEE Mains
Solution
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Correct option is B)

The lens is as shown in the figure.

To determine the radius of curvature of the curved surface, we can write


Solving this, we get


Speed of light in any medium is where and is the refractive index.

For the lens,


Putting these values in the lens makers formula for plano convex lens, we have,


Solving this, we get

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