How do you verify experimentally that the focal length of a convex lens is increased when it is kept in water? (AS1)

Aim: To verify that the focal length of a convex lens is increased when it is kept in water.

Materials required: Convex lens, tumbler, water, circular lens holder, stone

Procedure:

• Take a convex lens of focal length & a tumbler whose height is known and is four times the focal length of lens.

• Put the stone into the tumbler. Pour some water upto the level where the height of level should be greater than the focal length of lens.

• Now dip the lens horizontally using the lens holder.

• Set the distance between the lens & the stone such that the distance is equal or less than the focal length of the lens.

• Look at the stone through the lens. The stone is visible at this stage.

• Now slowly increase the distance between the lens & stone.

• As soon as the distance gets greater than the focal length, the stone will be not visible anymore.

Conclusion: If the lens is dipped to a certain height which is greater than the focal length of lens in air, the image is clearly visible. Thus the focal length of lens is increased in water.