How to Test Glass With a Laser Pointer

The most important optical properties of glass are determined by its index of refraction.

Laser pointers are as convenient as it gets. They put out a straight, coherent, monochromatic beam of light from a package as small as a writing pen. In the optics lab, there are many uses for a monochromatic beam of light --- including tests to characterize glass. The most important optical property of glass is its index of refraction. The index is a measure of the speed of light within the glass --- but that, in turn, determines how sharply a piece of glass will bend a beam of light.

Things You'll Need

• Laser pointer
• Flat piece of glass
• Protractor

Instructions

1. 

   You don't need a lot of fancy equipment to make some fundamental optical measurements.

   Identify the wavelength of the laser pointer. For a quick measurement of the index of refraction, the wavelength doesn't need to be known to great precision. If the specification sheet, for example, says it's a 640 nm (640 billionths of a meter) laser pointer, that's accurate enough.

2. Aim the laser "straight" at the flat side of a piece of glass. There are several ways you can do this. One easy one would be to place a piece of graph paper on the table and line up the laser so it went straight down a line on the graph paper. Then line up the glass to a perpendicular line on the graph paper.

3. Secure the laser so it doesn't move. You can rest it underneath a book, tape it to a table or put it in a laboratory mount.

4. Rotate the piece of glass so that it makes an angle of 80 degrees with the laser beam. That is, turn the glass so that the laser beam is striking the surface 10 degrees away from perpendicular. This is the input angle.

5. Measure the angle the laser makes within the piece of glass. This is the output angle. Put the protractor's flat edge against the flat side of the glass, and read the angle on the protractor where the laser beam passes underneath. There should be enough scattered light within the glass to let you do this, but you may need to darken the other light sources in the room.

6. Rotate the glass and measure the output angle for different input angles, such as 20 and 30 degrees.

7. Calculate the index of refraction. The index of refraction is given by the formula:

   sin(input angle) / sin(output angle). You will have three separate measurements, where the input angle is 10, 20, and 30 degrees. Each of those measurements of angle within the glass will give you the same measurement of the index of refraction.

8. Average the calculated values for index of refraction. This will decrease the effects of random errors within your measurement system. Index of refraction varies with wavelength, so you have just measured the index of refraction at the wavelength of your laser.

Tips & Warnings

• This introduction gives a conceptually and procedurally simple method for measuring the index of refraction. There are many other methods of varying complexity --- some of which can be done with a laser pointer.

• Although nearly all laser pointers should not cause permanent eye damage, looking directly at the beam from a laser pointer can cause temporary damage. Be careful.

References

• San Diego State University; Refraction

Resources

• Brooklyn College; Measuring Refractive Index