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Re: [Phys-l] refraction lab



There are a some problems with using the corners of various plastic containers for refraction experiments with liquids. Depending on your goal, the problems may or may not be important to you.

(1) Some liquids will attack some plastics.

(2) Molded plastic containers are often slightly wider at the top than bottom. This means the walls are not exactly vertical. In homemade glass prisms the walls also might not be exactly vertical, but if you follow the directions I published it's amazing how close the the walls can come to vertical, and it's also amazing how close to an equilateral prism you can come if you exercise reasonable care.

(3) I believe the 60-degree angle of an equilateral prism is easier to work with than the 90-degree angle of most containers. Commercially made glass and plastic prisms for holding liquids for refraction studies are generally equilateral prisms. I believe there is a reason for this. But they are expensive, and that's why I found a good way to make them myself. For one thing, setting the minimum angle of deviation is a bit easier and less error prone with the 60-degree prism compared to the 90-degree prism.

Here are some additional points...

[a] If you are not using the "minimum angle of deviation technique" you really ought to check it out. It works very well. When comparing different liquids, the minimum angle technique eliminates problems from not geting the prism in the same orientation each time. Indeed, you don't really want the prism in the same orientation for different liquids; rather, you want them adjusted for the minimum deviation for each liquid. You want to compare the minimum angle of deviation of one to the minimum angle of deviation of the other. If you don't do this, your results can be bogus. If two liquids have similar but different refractive indexes, and you don't use the minimum deviation technique, you can easily come to the wrong conclusion as to which liquid is more refractive.

[b] If your goal is to determine a reasonably accurate number for the refractive index of various liquids, you can easily get +- 0.001 with a homemade prism used with a spectrometer. If you don't use a spectrometer, but you measure distances and use trig, you can still get +- 0.01 for your refractive index.

[c] For really accurate refractive index measurements you do need to know the prism angle accurately. My earlier article (in The Physics Teacher) explains how to do this with a spectrometer. If you do not have a spectrometer, calibrate the prism using DI water at 20 Celsius, and use a sodium light source, or use a white light source and look at the yellow portion. At 20 Celsius and yellow light the refractive index of water is 1.333.

[d] I continue to suggest you try homemade glass/epoxy prisms. They're easy to make if you follow my instructions, and they work very well. You will not be sorry if you try them.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270
edmiston@bluffton.edu