Which is optically denser: water or air? Give a reason.

Water is optically denser than air.
Reason:
The optical density of a medium is related to the speed of light in that medium. A medium is considered optically denser if light travels more slowly through it.
• In water, light travels slower than in air because water has a higher refractive index compared to air. The refractive index nn is a measure of how much the speed of light is reduced in a medium.
• The refractive index of water is around 1.33, while the refractive index of air is approximately 1.0003.
Since the refractive index of water is higher, it means that light slows down more in water than in air. This makes water optically denser than air.
Final Answer:
Water is optically denser than air because light travels more slowly in water due to its higher refractive index.