Discussion on CBSE (Xth Board)

Clear materials have a refractive index ("n") which determines how much those materials can bend light. For a lens, the optical power and therefore the focal length of that lens depends on the ratio of the refractive indices of the two mediums (either of air and the lens or of water and the lens). For immersing a lens in water, as long as the index of refraction of the lens is greater than that of water (refractive index of water is about 1.34), the focal length will increase when the lens is immersed. The actual change in distance depends on the refractive index of the lens material.

If the refractive index of the lens was equal to that of water at about 1.34, obviously there would be no refraction and the focal point would be at infinity. If the refractive index of the lens was actually less than that of water, the immersed concave lens would bend light similar to how a convex lens would in air and the immersed convex lens would bend light similar to how a concave lens would in air