Question

Water is optically ______________ than glass.
A) rarer
B) denser
C) neutral
D) none of these

Answer 1

Hint: Light can travel in a vacuum and in a medium. The optical density of a medium is different from the physical density of a medium. The optical density of a medium is related to the speed of light in that particular medium. In an optically denser medium, the light will travel more slowly.

Complete step by step solution:
Step 1: Based on the differences in the speed of light in water and glass, determine their optical densities.
Light does not require a medium to travel. That being said, it can also propagate through a medium. The speed of light in a vacuum is considered to be its maximum speed and it is known to be $c=3\times {10}^8\text{m}{\text{s}}^{-1}$ . So naturally, the speed of light in a medium will be less than this value. The speed of light in water is more than its speed in a medium of glass. This suggests that out of the two mediums, glass and water, glass is the optically denser one.
So we can say that water is optically rarer than glass.

Hence the correct option is A.

Note: Alternate method
The refractive index of water is known to be $n_w=1\cdot 33$ .
The refractive index of glass is known to be $n_g=1\cdot 5$ .
The index of refraction can be considered as a measure of the optical density of a material as it is basically the ratio of the speed of light in a vacuum $c$ to speed of light in a medium $v$ i.e., $n={\displaystyle \frac{c}{v}}$ . So higher the value of the refractive index, higher will be the optical density of the material as then the speed of light in that material will be less.
Since $n_w<n_g$ , water will be optically rarer than glass.
So the correct option is A.