How to Build a Spectrometer with Just Three Household Items

Learning by doing is awesome. I kept this in mind while working on this year’s PhysicsQuest extension activities, and found some great hands on projects that make science fun for middle school students. Now my internship and time at the American Physical Society are coming to a close, so I want to share one of my activities with you! Inspired by a PopSci Five Minute Project, the cereal box spectrometer is easy to make, fun to play with, and a cleverly disguised optics lesson. Here’s how you make it:

Materials:
– Cereal box
– Compact disc
– Utility knife

Instructions:

-Cut a 1″ slit on the side of the cereal box just below the nutrition information.
-Cut a slit across the opposite side of the box and extend it 1″ on either side at an angle 45 degrees above the horizontal.
-Slide a CD into this slit.
-Make an eyehole on the bottom of the box below the CD.

Here’s how it works.
A CD has the very cool property of behaving like a reflective diffraction grating. An ordinary diffraction grating is a grid of tiny, evenly spaced opaque lines on an otherwise transparent material. Light can pass through the material but it has to bend around the lines, which are about the size of a wavelength of visible light. Different wavelengths of light bend at different angles, so light that is made up of more than one color gets “spread out” into its fundamental colors when it passes through a diffraction grating. This phenomenon is known as… diffraction! So, when light hits the CD it’s reflected because of the shiny coating, but essentially diffracted because of the way it reflects off the track of zeros and ones that spirals around the bottom of the disc. And that’s how the white light that enters the slit in the side of the box becomes the spectrum of colors you see when you look through the eyehole.

So what’s the point? Well, it turns out that there’s a lot more to white light than meets the eye (unless that eye is looking through our cereal box spectrometer). For example, it can explain why CFL bulbs are more energy efficient than incandescent bulbs. The following is an explanation from this year’s PhysicsQuest teacher’s manual:

Incandescent light bulbs work by running a current through a very long, thin coil of tungsten. This heats the coil up to a temperature of thousands of degrees Fahrenheit, and at that temperature the tungsten starts releasing energy in the form of light. It releases light of every color in the visible spectrum as well as infrared (IR) light, which we can’t see.

Because of the way they work, CFLs are more energy efficient and longer lasting than incandescent bulbs. They work by releasing electrons into the tube that makes up the bulb. The tube is filled with mercury gas, which reacts to the electrons by releasing ultraviolet (UV) light. The walls of the tube are coated in phosphors. When the UV light hits the phosphors, they emit visible light. Each phosphor only emits one color of light, though, so in order to achieve “white” light the CFL has to be coated with more than one phosphor. By using your spectrometer, you can see how many phosphors were used to make a CFL bulb and what color light they emit. Instead of a continuous spectrum (like what you see when you look at an incandescent bulb), you will see a line for each phosphor used in the CFL bulb.

You can build one too and try it out—it’s so much fun!

Physics Buzz blogger Quantum takes a homemade spectrometer for a test drive.

Fluffysingularity in her natural habitat.