Question

A concave mirror and convex lens are held in water. What change, if any, do you expect to find in the focal length of either?

Answer 1

Focal length of any mirror [ concave ] does not depends on medium , so it remains unchanged . if refractive index of a material of convex lens is greater than of water , then its focal length will increase .if refractive index of a material of concave lens is equal to water , then its focal length will become infinity if refractive index of a material is less than water than it behaves like a concave lens .

Answer 2

Case 1: (lens is in air)

Let air be medium 1 and glass (medium of lens) be medium 2.

From Snell's Law,

N21=V1/V2

[V1 means velocity of light in medium 1, V2 is velocity of light in medium 2]

=> Nga=C/Vg ---(1)

{C is speed of light in air}

Case 2: (Lens is in water)

Let medium 1 be water and medium 2 be glass.

Similarly, Ngw=Vw/Vg ---(2)

From (1) and (2),

Since C > Vw,

=> Nga > Ngw.

=> Extent/Degree of refraction (or bending of light rays) is greater in air. This means that focus o the lens will shift further away from the lens in water. This further implies that focal length of lens increases in water.