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.

Options

A. If both assertion and reason are true and the reason is the correct explanation of the assertion.
B. If both assertion and reason are true but reason is not the correct explanation of the assertion.
C. If assertion is true but reason is false.
D. If the assertion and reason both are false.

Answer

D

Explanation

If a mirror 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)mu_(g) - 1) ((1)/(R_(1)) - (1)/(R_(2)) ) `
`(1)/(f_(w)) = (""_(w)mu_(g) - 1) ((1)/(R_(1)) - (1)/(R_(2))) `
As `""_(w)mu_(g) lt ""_(a)mu_(g)`, hence focal length of lens in water inceases.
The refractive index of water is 4/3 and that of air is 1.
Hence, `mu_(w) gt mu_(a)`.