When light travels from a rarer to a denser medium, the speed of light in the medium:
A. Increases
B. Decreases
C. Remains the same
D. First increases and then decreases

Hint: The optical density of a medium is the ratio of the speed of light in vacuum to the speed of light in that medium. Hence, optical density is inversely proportional to the speed of light in the medium. Hence, in a medium of higher optical density (optically denser), light travels slower than in a medium of lower optical density (optically rarer).

Complete step by step answer:
The optical density of a medium is a measure of the speed of light in that medium. It is the ratio of the speed of light in vacuum (or air) to the speed of light in that medium. Hence, it can be said that the optical density of a medium is inversely proportional to the speed of light in that medium.

Hence, in a medium of greater optical density (optically denser), light will travel slower than in a medium of lower optical density (optically rarer).

Therefore, when light travels from a rarer to a denser medium, its speed decreases.
Therefore, the correct option is B) decreases.

Note: Students must not confuse optical density with the volumetric density of a substance. It is possible that a medium has a higher optical density but lower volumetric density than another medium and vice versa.
For example, water is denser than kerosene oil and kerosene floats on water, however, kerosene is optically denser than water.
When light travels from one medium to another, its speed and wavelength changes, but the frequency remains the same. The factor of change of the speed and wavelength is the same.