Aim : To prove that the focal length of a convex lens is increased when it is kept in water.
Apparatus: Convex lens of known focal length, circular lens holder, tall cylindrical glass tumbler, black stone, water.
i) Take a cylindrical glass tumbler whose height is much greater than the focal length of the lens and fill it with water.
ii) Keep a black stone at the bottom of the vessel.
iii) Now dip the lens into water using circular lens holder such that it is at a distance which is less than or equal to focal length of the lens in air.
iv) Now see through the lens to have a view of the black stone.
v) Now increase the height of the lens till you are not able to see the stone’s image.
vi) When the lens is dipped to a height which is greater than the focal length of lens in air, we are able to see the image. Showing that focal length of the lens has increased water.
vii) From this we conclude that the focal length of a convex lens is increased when it is kept in water.