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27 March 2009

Snel's Law demonstration


Let's get this out of the way right now: I'm not sure whether it's "Snel's Law" or "Snell's Law." My Serway credits the law to some dude (or dudette, I can't tell) named "Willebrørd Snell." However, I've seen two recent articles in The Physics Teacher and the American Journal of Physics which suggest that Snell should be spelled Snel. This is all an interesting debate, but in the end, who cares -- my purpose is to teach the law that bears the name.

To the right you see a fancy-pants refraction tank, sold by nada scientific for $200. If you have $200 lying around, it looks like a great tool -- the 6V light mounts anywhere around the circular tank, allowing you to send light either from air to water or from water to air. I don't know about the durability -- I had a similar tank whose stand broke off after only a few years of use.

Thing is, even though I do have $200 lying around, I prefer to spend money on other things. Refraction demonstrations can be done much more easily and cheaply. A fishbowl with a laser pointer works just fine -- use chalkdust to visualize the laser in the air, and a teeny drop of milk or coffee creamer to visualize the laser in the water.

The question becomes, how do you make quantitative measurements of incident and refracted angles for a laser entering water in a fishbowl? A dry erase marker will write and erase on glass. I have a student trace the incident and refracted rays with the dry erase marker. Then, I turn the lights back on, and use a protractor to measure angles of incidence and refraction. Snel's law always seems to be verified.

GCJ


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