the average speed of light slows to 0.75 when it refracts through a praticular piece of plastic.

Question

a. what change is there in the light's frequecny in the plastic?
b. in its wavelength?

Answer 1

You must mean that the speed of light slows to 0.75c in the plastic, not 0.75

a) frequency does not change
b) wavelength is shortened to 0.75 times the original wavelength (in air).