DMAPT Optics – CD Diffraction

The Detroit Metropolitan Area Physics Teachers will be meeting at the University of Michigan today (Feb. 3, 2007). We are being hosted by the FOCUS Center.

As a part of the meeting we, the physics teachers will be sharing our own tips, labs, and demos with the other teachers as well. I will be sharing a cool use for old CDs. You know, the ones you used for backups and don’t need anymore or ones that didn’t burn correctly.

CDs use a diffraction grating to align the laser with the data on the disk. This is why we see rainbows on the bottom of the disks. You can use this property to make a simple spectroscope. A spectroscope allows you to see the spectrum of light you’re looking at. You’ve probably seen a demonstration of a prism separating white light into a rainbow of colors. A spectroscope will do the same thing. The fun part is that all lights do not give off the same spectrum. You will see distinct bands rather than a continuous spectrum.

There are several links on the web showing you how to make a CD Spectroscope, my favorite is one by Alan Schwabacher at the University of Wisconsin-Milwaukee.

I’ve but a few of these together. In general the more time you spend the nicer it will look, but you only need about 5 minutes to make a functional spectroscope capable of illustrating the differences between incandescent and fluorescent lights.

Another fun thing to do with old CDs (or DVDs) is to experimentally determine to groove spacing. Diffraction Gratings work because they have a series of fine grooves, too small for us to see. CDs are not truly diffraction gratings as they have no grooves, but actually have lines of pits that effectively act as a grating. The colors are separated due to interference. At smaller angles the shorter wavelengths of light will constructively interfere so we see violets and blues while at higher angles we see constructive interference in the longer wavelengths.

In order to determine the groove spacing you need only an old CD a single point light source (light bulb) and a ruler. The whole activity is outlined in a guide put together by General Atomics. You’ll want the section on the Electromagnetic Spectrum, specifically lab 3b.

In the basic setup you want the light behind you. You hold the CD up and try to center the reflection of the light in the center of the CD (i.e. in the hole so you can’t see it). Then while keeping the reflection in the hole you slowly bring the CD closer to your eye until you see a circular rainbow extending around the whole CD (you have to get it pretty close to your face, so your head may obscure part of the light). You then simply measure the distance from the CD to your eye and the distance from the center of the CD to a band of color that you know the wavelength for. In the General Atomics activity they use 450 nm for violet. I will often use red (620 nm) as well. You then plug the data into the formula and you get your answer.

Another way you can do this activity is to peel the label off the disk. I’m not very good at this, but I know it can be done. You can then look through the CD at the light. Again, you center the light in the hole and bring the disk towards your face. Sometimes when you buy a stack of CDs they will include one with no label. The picture below was taken through one of these.

You can even see where there are some scratches on part of the disk. Interestingly the violet band does not show up in the picture. I could see it, but the ccd in my camera failed to capture it.

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