Question

An empty test tube, when placed in water in a beaker, shines like a mirror when seen at certain angles. Give relevant reason with an appropriate diagram.

Answer 1

When a test tube is immersed at a certain angle in a beaker of water appears to have a shiny surface for the following reasons:
1. Consider the rays of light moving in water. They get refracted as the medium through which they move changes.
2. Some of these rays reach our eye and we perceive the brightness of water depending upon the amounts of light reaching our eye.
3. When there is a test tube in water, rays of light moving in water also fall on the interface between water and wall of the test tube.
4. The rays that fall at an angle greater than the critical angle of water (48 degrees) undergo total internal reflection.
5. When such rays come out of the water and reach our eye, we get more light than usual in emergent ray's direction.
6. To us, the extra light seems to be coming from the wall of the test tube and therefore we find brighter than the surrounding water.