Additional Notes on Refraction: The wavelength changes!

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

lin water= 633 nm / 1.33 = 476 nm


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.