Since light waves entering a material with a higher index of refraction slow down, the wavelength shortens. (Frequency stays the same!) This is illustrated on page 22 with an analogy to rows of marchers moving from a hard pavement to a muddy field. For waves, it can be shown that the index of refraction in a medium with index of refraction "n" is given by l/n. That is, the wavelength in a vacuum is divided by the index of refraction of the medium.
Since n is always greater than 1, the wavelength in a medium is always less than in a vacuum.
EXAMPLE:
The red light from a Helium Neon
laser has a wavelength of 633 nm in a vacuum. What is the wavelength
in water?
Solution: From the table on page 6, the index of refraction of water is 1.33
Does this mean that the color of the laser changes when viewed from
under water? No, because 1) the interaction between light and the
eye's color sensors is a photon interaction and the frequency of the
photon does not change and 2) the water is external to the eye's
sensing system- the material surrounding the sensors on the retina
doesn't change.
The index of refraction of a material is one of the physical properties that allows a chemist to identify it. A refractometer is an instrument used to measure index of refraction.