Question

For visible light if medium changes then which quantity remains unchanged

Answer 1

Light travels as waves, with the wave-fronts perpendicular to the direction of motion. As light moves from air into water or glass, it not only slows, but the wavelength changes. The wavelength becomes shorter in the denser medium. Interestingly, the frequency of the waves does not change as the light moves from air to a denser medium - only the number of waves per unit time changes.
he light ray will change its direction through the next medium, or appear to `bend'. This is called refraction of light. It is important to note that while the speed of the light changes when it passes into the new medium, the frequency of the light remains the same.

V=frequency× wavelength.

As frequency doesnt change so speed and wavelength change to balance the equation.