IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# Lenses from Beaker

## Introduction

A round glass piece which is thick at the centre and thin at the periphery is picture of the convex lens we have in our mind. Similarly a round glass piece which is thin at the centre and thick at the periphery is the picture of a concave lens we have in our minds.

In this demonstration we try to redefine this narrow picture of lenses and give a broader perspective of this concept of lens. The use of milky water and smoke makes the path of light visible by scattering the light that makes the demonstration very beautiful.

## Apparatus

A plastic box with a lid, incense stick, milky water, a 100 ml beaker, two laser torches.

## Procedure

1. Light the incense stick and fill the plastic box with the smoke of the incense stick.
2. Tie the two laser torches with a rubber band and make them on by pressing their switches. Check whether you can see two parallel beams of light in the smoke filled box.
3. Fill the beaker with milky water and put it in the middle of the smoke box. See that the parallel beams of light get converged when they pass through the beaker. The beams of light meet at a point and then continue their journey forward.
4. Now take out the milky water from the beaker and pour it in the plastic box. Add some more milky water to fill the plastic box to half of its capacity.
5. Fill the empty beaker with smoke and put a lid on it.
6. Hold the empty beaker filled with smoke in the milky water in the centre of the box.
7. Again shine the two parallel beams of light on the beaker. See that the parallel beams of light get diverged when they pass through the beaker.

## Discussion

Water is optically denser than air. The parallel beams of light go from a rarer medium to a denser medium when they pass from air in the plastic box to the water filled beaker. The beaker with its round surfaces and filled with water acts as a convex lens. So the beams of light get converged to a point which is focus of the convex beaker lens.

When the water is filled inside the box and the beaker is kept empty, the parallel light beams go from the denser medium of water in the box to the rarer medium of air inside the beaker. This beaker with its round surface and filled with air and smoke acts as a concave lens. So the beams of light get diverged when they pass through the beaker. These diverged beams seem to come from behind the beaker. This point is the virtual focus of the concave beaker lens.

So we see that the same beaker can act as both a convex as well as concave lens. The type of lens is determined by the medium inside the beaker and the medium surrounding it.