Light of wavelength 500 nm in a vacuum enters glass of refractive index 1.5. Inside the glass its wavelength becomes

Explanation

Frequency is unchanged on entering a medium, and since the speed falls by the factor of the refractive index the wavelength must fall in the same proportion, giving 500 divided by 1.5, which is about 333 nm. The colour perceived does not change, because it is determined by frequency. Multiplying instead gives the 750 nm distractor.