Question

Out of air and glass which is optically rarer? Give reason.

Answer 1

Hint The optically rare or dense medium is categorized based on the velocity of light in the medium. Light travels comparatively slowly in optically dense medium and travels fast in an optically rare medium

Complete step by step answer
An optically rare medium is one where the velocity of light is faster than the velocity of light in an optically dense medium. When light enters an optically dense medium from an optically rare medium, it changes its direction and is refracted towards the normal to the surface. Contrarily, when light enters an optically rare medium from an optically dense medium, it is refracted away from the normal.
We know that the speed of light is $ 3 \times {10^8}\,m/s $ in the air while it is roughly $ 2 \times {10^8}\,m/s $ in a glass. So, we can say that air is optically rare than glass. The reason for this distinction is due to the velocity of light being higher in the air than it is in water.
The reason for the velocity of light being higher in the air than in glass is due to air having a lower refractive index than glass. As the speed of light is inversely proportional to the refractive index of the medium, air, which has a lower refractive index than glass, will allow light to propagate more quickly than in glass.

Note
Here we have assumed that the composition of air and glass is constant and hence the speed of light is constant in the medium. However, the speed of light changes mildly depending on the composition of air and glass. Generally, though, light still travels faster in the air than in glass.