PHY 2049L
Lab 8

Snell's Law

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Updated: 8/09/2009
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Prelab

Answer questions 1 through 4 on page 407 of the lab manual.

Comment

The procedure, data tables, and post-lab questions are all given on a handout. The handout also emphasizes the basic principles of safety when working with lasers, points I will elaborate on at the start of the lab.

Link to pdf of first 3 pages of the handout.
The full handout with data table and questions will be available in the lab.

Safety:
The main principle of safety around equipment like the lab lasers we use is the same as for guns: You should treat every gun as if it were loaded, even if you unloaded it yourself; you should treat every laser as if it were dangerous even if you have checked the label stating that its output is in the "safe" region below 1 mW. You want to get in the habit of working with care so that skill will be automatic if you are working with lasers that are more hazardous. My favorite summary of the issue is given on an unofficial warning sign in a lab:

Do not look into laser with remaining good eye.

Apparatus

This lab probably uses the most primitive tools (apart from a pretty good laser) of all of the experiments done this semester:

water tub, laser, and measurement tools

The equipment consists of a water tub, laser, and measurement tools. You will also have a sheet of newsprint to record the positions of the incident and refracted rays. Pretty basic. However, despite the simplicity of the equipment, one can obtain very good results if careful measurements are made. Better yet, there is nothing hidden inside a digital meter; everything in this experiment is right in front of you.

The way this will look when doing the first part of the experiment (measurement of the index of refraction of water and verification of Snell's Law) is diagrammed below:

layout of the experimental measurement

The critical parts of this measurement can be seen in this diagram: alignment of the tub parallel to the main baseline (the one running vertically, the only one drawn before doing the observations), a single entry point for every ray that is measured, and maximizing the leverage on the angle measurement by having the laser as far from the tub as the paper will allow. You will not be able to see the path of the laser beam (shown as a red line), just the two spots where it enters and leaves the tank. The point where the beam leaves the laser can be located by putting a pencil vertically in the beam.

The case in this diagram is for the first observation, which will be for the largest angle to be measured. The other positions should be (roughly) equally spaced between this angle and zero degrees.

We will also look at total internal reflection during this lab. I will show you how to use water to make a "light pipe", since this is not a demo we can do very easily in the classroom, and you will estimate the critical angle for comparison to the one you calculate from your data.

 
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?? Contact me if you have any questions.