The frequency of a light wave is the same when the light travels in ethyl alcohol or in carbon disulfide.

Question

Find the ratio of the wavelength of the light in ethyl alcohol to that in carbon disulfide.
The refractive indices n of carbon disulfide and ethyl alcohol are n(disulfide) = 1.632 and n(alcohol) = 1.362.

Answer 1

Of course the frequency is the same. There is no box to store extra waves in so the same number of waves have to pass you in a second in each medium.
Therefore:
the wavelength will be proportional to the speed in each medium.
v = c/n
so v in alcohol = c/1.362
and v in disulfide = c/1.632

so v al/vdis = 1.632/1.362