Question

Discuss how the focal length of a convex lens changes when it is immersed in to water.​

Answer 1

Answer:

When convex lens is in air, it’s focal length is given by

1/f(air)=(n-1)[(1/R1)-(1/R2)] (1)

Here, n is refractive index of material of lens relative to air.

R1 and R2 are radii of curvature of two surfaces of convex lens. Surface on the object side has positive radius of curvature and that opposite side has negative radius of curvature.

Now, when the lens is placed in water of refractive index n’ ,the refractive index of the lens is

1/f(water)=[(n-n’)/n’][(1/R1)-(1/R2)] (2)

It is clear from equations (1) and (2) that f(water)>f(air).

Answer 2

Thus, the focal length of the lens will increase. When any lens or glass slab is immersed in a liquid whose refractive index is equal to that of lens, refractive index of lens will be equal to 1 relative to that liquid. The focal length of the lens in this case, will tend to infinity. ⇒f=∞ .