Question

An empty test tube is kept in a beaker of water. Its surface inside the water shines like a mirror. Explain why?

Answer 1

Heya friend!!
Rajdeep here....

When an empty test tube is kept in a beaker of water, the surface of the test tube separates two mediums -- Air inside the test tube and water.

When a ray of light from water enters the test tube, its angle of incidence becomes greater than the critical angle for the pair of media, and total internal reflection occurs, due to which it appears shiny.

Refer to the above diagram... you will understand clearly.
Thanks!