# concave and convex lenses are placed in water what is channge in focal length?

Relevance

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.

• I'm not real sure on this. Physics class was a long time ago.

I think that the focal length will be affected by the index of refraction of the lens itself. (That is that part I am not sure of.)

If so, then the index of refraction(IR) will be different relative to the water instead of the air. This would make the focal length shorter in the water but I am not sure how much or how to figure it out.

• Bitu, when u place them in water, the change in focal length is given by the lens maker's formula which relates f to n2 and n1. While for glass in air, its ~1.5 and 1, for glass in water its ~1.5 and 1.33.

• there is no change in the focal length as the light rays do not undergo any refraction and ther is no rule which states thet the focal length of the mirror will chane when immersed in a liquid