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## Answers

The change in focal length can be found by the formula:

1/f = (n-1)(1/R1-1/R2)

where n is the refractive index of the material of the lens with respect to the refractive index of the medium.

that is, n = n(lens)/n(medium).

if n is greater than 1 the focal length will increase and if n is less than 1 the focal length will decrase.

The refractive index of water is 1.33. So, if the refractive index of the lens is less than 1.33, the focal length will reduce and we get a negative value for focal length, which indicates that the length will act like a diverging lens. If the refractive index of the lens is greater than that of water, the lens will act as a converging lens.Here also, the focal length will be reduced, as the refractive index of the water is greater than that of air. That is:

n(lens)/n(water)<n(lens)/n(water)