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CD Spectroscope

Science Snack
CD Spectroscope
Make a truth teller for light.
CD Spectroscope
Make a truth teller for light.

Turn an old CD into a spectroscope to analyze light—you may be surprised by what you see. Try pointing your CD spectroscope at the fluorescent light in your room, sunlit clouds in the sky, even your friend’s colored shirt to reveal the wavelengths of light that mix together to create the color you see!

Tools and Materials
  • A compact disc (CD)
  • A cardboard tube that's at least 12 inches long (approximately 30 centimeters) and 3 to 4 inches (7.5 to 10 centimeters) in diameter
  • Two covers for the cardboard tube—we suggest two flat pieces of cardboard large enough to cover each end of the tube, or you can also use the plastic covers that come with a cardboard packing tube.
  • Razor knife such as an X-ACTO knife
  • Tape
  • Access to fluorescent light
  • Saw
  • Cutting guide (scaled for a 3-inch tube)—PDF included
  • Access to a printer
Assembly
  1. To help you build your spectroscope, we’ve created a cutting guide (PDF included). Print the cutting guide and wrap it around your tube. We’ve scaled the guide for a standard 3-in packing tube. If needed, you can scale the guide to ensure it wraps around your tube without a gap or overlap (see below).
  2. Use a saw to cut the tube at an angle along the curved line on the cutting guide (see below). The cut will make the CD tilt at an angle approximately 30 degrees from the end of the tube.
  3. Use a razor knife to cut the rectangular viewing hole—the black square on the cutting guide. You can remove the cutting guide now (see below).
  4. Next, cut a clean slit less than 1 mm wide and 5 cm long in one of pieces of flat cardboard (or plastic tube cap). Tape the flat cardboard onto the end of the tube furthest from the CD—the top of the tube. Hold the tube as shown below and align the slit horizontally.
  5. Tape the second flat piece of cardboard (or plastic tube cap) over the bottom end of the tube, behind the CD, to exclude any stray light.
  6. Insert the CD into the CD slot, so that it reflects the light coming through the top slit into your eye.
To Do and Notice

Hold the tube upright and point the top slit at a fluorescent light.

Press your eye to the viewing hole.

On the CD, look for a clear, solid line of light broken up into colored bands: this is the spectrum of light reflected from fluorescent light onto the CD.

Adjust the angle at which you look through the viewing hole at the CD to find the best view of the light spectrum.

Notice that the fluorescent light produces bright lines. The bright lines are the spectrum of mercury gas inside the tube. An incandescent light, by comparison, makes a continuous spectrum.

What’s Going On?

All light is made up of waves of electromagnetic radiation. A spectroscope spreads each different wavelength to a different position within a spectrum of light. 

Music is digitally recorded as circular tracks of ones and zeros on the mirrored surface of a CD. These circular tracks are so close together that they can act as a diffraction grating for light.

When the light enters the tube, it is spread into a spectrum perpendicular to the CD tracks. This is why the slit and the viewing hole are located at right angles.

Each color bends at a particular angle. For you to see the spectrum, the light must diffract off the CD and reflect into your eye. Adjusting the tilt of the CD allows you to properly bounce the spectrum into your eye.

Going Further

Examine the spectra produced by other light sources, such as light-emitting diodes (LEDs), sodium-vapor streetlights, and neon tubes.

When you look at an incandescent light and a fluorescent light by eye they might appear to be the same shade of white, yet looking at them with a spectroscope reveals that they are composed of two totally different spectra.

In 1788, Comte de Buffon said that he was sure we would never know what the sun was made of. You can look through your spectroscope and prove him wrong. Be careful not to look directly at the sun, but you can use the spectroscope to look at sunlight reflected off white clouds, white walls, or white paper to see the spectrum of the sun. If you make an excellent top slit on a long tube (24 in or 60 cm) you may even see thin dark lines in the solar spectrum; these are called Fraunhofer lines and reveal what the sun is made of.

In the 1860s, William Huggins discovered what the stars were made of by viewing them through a spectroscope.

Your CD spectroscope connects you to a long history of discovery!

This Science Snack is part of a collection that showcases LGBT artists, scientists, inventors and thinkers whose work aids or expands our understanding of the phenomena explored in each Snack.

Highlighted Scientist: Rebecca Oppenheimer
Source: Jörg Meyer for Columbia College Today. Used with permission.

Dr. Rebecca Oppenheimer (pictured above) is a comparative exoplanetary scientist; she studies planets that are outside of our solar system. She has been a leader for many projects relating to exoplanet study, including the Advanced Electro Optical System (AEOS) Telescope in Maui, Project 1640 at the Palomar Observatory, and a starlight suppression system for the International Gemini Observatory Planet Imager project (GPI), which was conducted in her own lab. Dr. Oppenheimer's optics laboratory in the Rose Center for Earth and Space is the birthplace of various new astronomy-related devices that have led to the ability to directly see and take spectra of nearby solar systems. With the CD Spectroscope Science Snack, you can see for yourself different colors and wavelengths of light all around you.