In full early-90s nostalgia mode, you skate down the street in your roller blades. Your thrill at the excitement of the open road distracts you, and one foot goes off the pavement into the soggy soil beside the road. That foot immediately slows down due to the drag and as a result of the difference in speed between the foot on the road and the foot on the dirt steers you right off the road where I hope you don’t hurt yourself.
Refraction is the same thing, but with light.
Pass from one medium to another in which light travels at a different speed, and the light will bend. Students grumble about this sometimes. Isn’t the speed of light a universal constant, after all? Well yes – sort of. When passing through a material however, the interaction with each successive atom takes time and that in effect slows down the average speed. In fact while it’s impossible to exceed the speed of light, it’s perfectly possible to exceed the average speed of light when it’s being slowed down in a material. Cherenkov radiation is one example.
Different material slow down light by different factors. We quantify this with what we call a refractive index. Diamond has a fairly high refractive index of about 2.4, so light travels at a speed of c/2.4 in that material. This is partially responsible for the beautiful optical effects of a well-cut diamond.
What’s more interesting is the fact that the refractive index is generally not a constant. Within the same material it can be different for different wavelengths. Usually – but not quite always – higher frequency light experiences a higher refractive index. It gets bent harder. It’s not obvious at the level of classes I teach why this should be so. Since the details aren’t really the key issue, the main thing to remember whether it’s high frequency or high wavelength that gets bent most strongly. It can be difficult to remember, until I remind the class about this:
And it’s been my experience that in 2009 – a full thirty-six years after its release – everyone recognizes it. And in my sections no one ever misses any question involving refractive index as a function of wavelength.