EDIT: title should be Is colour dependent on wavelength or frequency Okay, and so here is the question in my mind. We all know (or at least taught) that when light enters an optically denser medium, it slows down and its wavelength shortens. Frequency, however, remains the same. Going by the above, will we see (or at least theoretically conclude) that there is a change in colour observable? If colour is determined by wavelength, then the above should see a blue-shift. If colour is determined by frequency, then the above should see no change. What is the correct answer?