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Assertion : A concave mirror and convex ...

Assertion : A concave mirror and convex lens both have the same focal length in air. When they are submerged in water, they will have same focal length.
Reason refractive index of water is smaller than be refractive index of air.

A

If both the assertion and reason are true and reason explains the assertion.

B

If both the assertion and reason are true but reason does not explain the assertion.

C

If assertion is true but reason is false.

D

If assertion is false but reason is true.

Text Solution

Verified by Experts

The correct Answer is:
D
If a mirrorr is placed in a medium other than air, its focal length does not change as `f=R//2`. But for the lens
`(1)/(f_(a))=(._(a)n^(g)-1)((1)/(R_(1))-(1)/(R_(2)))`
and `(1)/(f_(w))=(._(w)n^(g)-1)((1)/(R_(1))-(1)/(R_(2)))`
As `._(w)n^(g)lt ._(a)n^(g)` , hence focal length of lens in water increases.
The refractive index of water `4//3` and that of air is 1. Hence, `mu_(w)gtmu_(a)`
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