Question

Why frequency of light remains constant if it travels from one medium to another as its wavelength and frequency change.

Answer 1

Hope the above answer helps

Answer 2

Frequency of light is a source dependent quantity. For example, once light of a frequency of 4 x 10∧14 Hz has been emitted, its actual frequency will not change

So if it travels from one medium to another,  as the velocity changes, the wavelength changes but frequency remains constant.