When light travels from water to air, which property remains unchanged?

Correct Option: Frequency

Explanation:

When light travels from one medium to another (such as from water to air), its speed and wavelength change due to the change in optical density of the medium.

However, the frequency of a wave depends exclusively on the source emitting the light. Since the source of light remains the same regardless of the medium through which the light propagates, the frequency remains unchanged (constant) during refraction.

The relation between speed ($v$), frequency ($f$), and wavelength ($\lambda$) is given by:

$v=f\times \lambda$

As light enters a different medium, its speed $v$ changes, which causes a corresponding change in the wavelength $\lambda$ such that the frequency $f$ stays constant.