Which of the following does not change when light travels from one medium to another?

Correct answer: (c) Frequency

Explanation

During refraction, the speed and wavelength of light change as it passes between media of different optical densities.

The frequency stays constant because it is fixed by the source, so the number of waves per second is unchanged.

The refractive index differs from medium to medium, governed by Snell's law (sin i / sin r = constant).