Aim : To verify experimentally that the focal length of a lens depends on its surrounding medium.
Apparatus : Convex lens of known focal length, circular lens holder, tall cylindrical glass tumbler, black stone, water.
Procedure :
(1) Take a cylindrical glass tumbler whose height is much greater than the focal length of the lens and fill it with water, such that the height of the water level from top of the stone is greater than focal length of the lens.
(2) Keep a black stone at the bottom of the vessel.
(3) 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.
(4) Now see through the lens to have a view of the stone.
(5) Increase the height of the lens till you are not able to see the stone's image.
(6) 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. It shows that focal length of the lens is increased in water.
(7) Thus we conclude that the focal length of a lens depends on its surrounding medium.
