Question

If the lens you tested were placed underwater (ηwater = 1.33) and you measured the focal length again, should it increase, decrease, or remain unchanged?

(Explain why, conceptually) By what factor would f change by?

Answer 1

When a lens is immersed in water, the focal length increases.

The relative refractive index of glass with respect to water is less than that of glass with respect to air.

[ Let n be the refractive index of glass, then

  

where 1.33 is the refractive index of water and 1 is the refractive index of air.]

Hence, the bending of light at the interface of glass- water is lesser and hence a parallel beam of light falling on the lens is bent less to meet at point on the principal axis. So the lens has more focal length in water than in air.

[Refractive index for lens immersed in water is less than that of lens in air, so according to the refractive index value, focal length will shift. ]