Spectrometer

Spectrometers are used in chemistry and astronomy to measure light. In astronomy, we can find out about distant stars without ever traveling to them, because we can split the incoming light from the stars into their colors (or energies) and “read” what they are made up of (what gases they are burning) and thus determine their what they are made of. In this experiment, you’ll make a simple cardboard spectrometer that will be able to detect all kinds of interesting things!

SPECIAL NOTE: This instrument is NOT for looking at the sun. Do NOT look directly at the sun. But you can point the tube at a sheet of paper that has the sun’s reflected light on it.

Experiment & Video

Find an old CD and a cardboard tube at least 10 inches long.  Cut a clean slit less than 1 mm wide in an index card or spare piece of cardboard and tape it to one end of the tube.  Align your tube with the slit horizontally, and on the top of the tube at the far end cut a viewing slot about one inch long and ½” inch wide.  Cut a second slot into the tube at a 45 degree angle from the vertical away from the viewing slot.  Insert the CD into this slot so that it reflects light coming through the slit into your eye (viewing slot).

Aim the 1 mm slit at a light source such as a fluorescent light, neon sign, sunset, light bulb, computer screen, television, night light, candle, fireplace… any light source you can find.  Look through the open hole at the light reflected off the compact disk (look for a rainbow in most cases) inside the cardboard tube.

What's Going On?

Usually you need a specialized piece of material called a diffraction grating to make this instrument work, but instead of buying a fancy one, why not use one from around your house?  Diffraction gratings are found in insect (including butterfly) wings, bird feathers, and plant leaves.  While I don't recommend using living things for this experiment, I do suggest using an old CD.

CDs are like a mirror with circular tracks that are very close together. The light is spread into a spectrum when it hits the tracks, and each color bends a little more than the last. To see the rainbow spectrum, you’ve got to adjust the CD and the position of your eye so the angles line up correctly (actually, the angles are perpendicular).
You’re looking for a spectrum (the rainbow image at left) - this is what you’ll see right on the CD itself. Depending on what you look at (neon signs, chandeliers, incandescent bulbs, fluorescent bulbs, Christmas lights...), you'll see different colors of the rainbow.

Troubleshooting: This is a quick and easy way to bypass the need for an expensive diffraction grating. Use your spectrometer to look at computer screens, laptops, night lights, neon lights, candles, campfires, fluorescent lights, incandescent lights, LEDs, stoplights, street lights, and any other light sources you can find, even the moon through a telescope. 

I hope you're having fun. See you in the next issue!

Warmly,
Aurora