Remember that the frequency does not change in a medium. Only the wavelength changes. The easiest way to conceptualize this is to think about the fact that if light is striking a surface of, say, water, the crests will hit the surface with a frequency. Inside that water, those crests have to be transmitted at the same rate, since the water has to "process" that number of crests no matter what.

In other words, its like you are scrunching the wave. It travels slower, but at both interfaces it has to "process" the same number of crests every second.

In mathematical terms, that means if n gets larger, the value of v = f*wavelength must get smaller. Since only wavelength changes, the wavelength is reduced.

Another way to think about it is that frequency corresponds to the energy of the light particle. The energy, and thus the frequency, won't change when you switch mediums.