One Beam Becomes Ten!? Exploring Laser Interference with a Recycled CD
I’m Ken Kuwako, your Science Trainer. Every day is an experiment.
Wouldn’t you be surprised if the CD you use to listen to music was actually a sophisticated “light experiment device”? Now that streaming is the norm, the CD collecting dust on your shelf is a treasure trove that lets you easily experience the wonders of physics. Today, I’ll introduce a magical experiment where you use a CD to turn a single laser beam into multiple bundles of light.
If you tilt a CD, it sparkles in rainbow colors, but if you look closely, the CD itself isn’t colored. In fact, the surface of the CD has tiny, invisible “grooves (tracks)” lined up. The light reflected there undergoes a phenomenon called “interference,” which is what reaches our eyes as a rainbow.
Today, we’re going to try an experiment to observe light interference by extracting a component called a “diffraction grating” from that CD. I was honestly surprised by the profound nature of the physics when I tried this myself after being taught by Mr. I here at school. I’ll also share a hint on how to build a “spectroscope” for splitting light.
Science Recipe: How to Extract a Light-Splitting Diffraction Grating
What You’ll Need
CD-R (blank or an unwanted/used one), Scissors
Experiment Method
The experiment is very simple, but it requires a little force.
First, cut the CD with sturdy scissors. CDs are surprisingly tough, so we strongly recommend using “tin snips” or similar tools to prevent the scissor blades from chipping.
Cut it out in a fan shape, like this. Be careful not to cut your hands, and protect your eyes, as plastic shards may fly off.
Once cut, remove the silver coating (reflective layer) from the surface. If you stick clear tape (like cellophane tape) to it and peel it off, only the transparent plastic part will remain. This transparent plate is where the CD’s essence—the “invisible grooves”—is left behind.
Shine a laser beam onto this transparent fragment. For safety, please use a commercially available laser pointer.
Experiment Results: Why One Light Beam “Multiplies”
Shine the laser on it, and voilà! The single beam of light that was traveling straight is now projected as multiple beams to the left and right.
We also tried it with a green laser.
Why does the light multiply? It’s because the countless tiny grooves on the CD scatter or transmit the light waves. The separated light waves “reinforce” each other in some places and “cancel out” each other in others, causing strong light to shoot out only in specific directions. This phenomenon is called light interference.
By applying this mechanism, you can create a tool called a “spectroscope,” which is used to analyze the colors mixed within light. If you look through this CD fragment at sunlight or fluorescent light, you should see a beautiful rainbow staircase.
While observing is fun enough, you’ll uncover a deeper world of science if you approach it with questions like, “Why does the light split?” or “What happens if the groove spacing changes?” Use a common CD to enjoy the artistry of light!
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