When monochromatic light travels from one medium to another, its wavelength changes but its frequency remains the same. Why?

When light passes from one medium to another medium, which one of these quantities, remains unchanged.

When light travels from air to water, which parameter does not change?

In these question, a statement of assertion followed by a statement of reason is given .Choose the correct answer out of the following choices: Assertion and reason both are correct statements and reason is correct explanation for assertion. Assertion and reason both are correct statements but reason is not correct explanation for assertion. Assertion is correct statement but reason is wrong statement. Assertion is wrong statement but reason is correct statement. Assertion: the frequencies of incident ,reflected and refracted beam of monochromatic light incident from one medium to another are same. Reason: The incident, reflected and refracted rays are coplanar.

A ray of monochromatic light travelling in vacuum with speed C, wavelength X and frequency u, enters into a medium of refractive index 1.5 . What will be its new speed, wavelength and frequency ?

A ray of monochromatic light travelling in vacuum with speed c ,wavelength gamma and frequency v,enters into a medium of refrative index 1.5 .What will be its new speed ,wavelength and frequency?