Explanation:

When light passes from one medium to another, the physical quantity that remains unchanged is the frequency of the light wave.

Frequency:
Frequency is the number of complete cycles of a wave that occur in one second. It is measured in hertz (Hz). The frequency of a wave determines its color in the visible spectrum and its position in the electromagnetic spectrum.

Velocity:
Velocity is the speed at which light travels in a medium. The velocity of light is different in different media. When light passes from one medium to another, its velocity changes according to the refractive index of the media.

Wavelength:
Wavelength is the distance between two corresponding points on a wave. It is usually measured from crest to crest or trough to trough. The wavelength of light is also influenced by the refractive index of the media it passes through.

Refractive Index:
When light passes from one medium to another, it changes direction due to the change in velocity. This change in direction is known as refraction. The degree of refraction depends on the refractive index of the medium. The refractive index is defined as the ratio of the velocity of light in a vacuum to the velocity of light in the medium.

Conclusion:
When light passes from one medium to another, the frequency of the light wave remains unchanged. This means that the number of complete cycles of the wave that occur in one second does not change. However, the velocity and wavelength of the light wave change according to the refractive index of the media.