I am totally stuck over this concept and google doesn't help.
First, We say that light's speed is constant for all colors in vacuum but different in all other media. Then which color's speed are we referring when we say that light's speed in that medium (like glass) is c divided by refractive index? Is it average of all or what?
Second, why do we say that frequency of light is constant when it changes media when we know that from red to violet, we have different frequencies and wavelengths (e.g. for violet it's 680-790 THz and 405-480 THz for red)?