Monday, November 3, 2008

Make your own spectrometer with a CD

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Have a spare CD or two? I'm sure you have it. Then let's make a spectrometer out of it. A spectrometer is device that is used to determine the chemical composition of a material by measuring the electromagnetic spectrum of the light emitted by that material when incandescent. By identifying the spectral lines produced by the light, it's possible to determine the composition of the material. Spectrometers are often used in astronomy to determine the composition of stars and distant galaxies.

To make a simple spectrometer you will need a CD or DVD, a cardboard box like a cereal box and some tapes. Cut a narrow slit (about a millimeter or less) at one end of the box and place the CD at the opposite end at an angle of about 45 to 60 degree.

spectrobox3

When viewed through the eye hole, you will see a spectrum of the light entering through the slit. Adjust the angle at which you place the CD so that it gives the best view of the spectrum. Use a pressed compact disc, rather than a blank disc, for best results.

spectrobox cerealboxspectrometruse300

How does this happen?

A compact disc, CD or DVD, stores information in bits of zeros and ones in circular tracks. These tracks are so close together that they act as a diffraction grating for light. That's why you see colorful rainbows when you hold a CD to light.

Now test your CD Spectrometer to various light sources. Some light sources to try out with are:

Incandescent light

An incandescent light has continuous spectrum with all visible colors present. There are no bright lines and no dark lines in the spectrum - a typical blackbody spectrum.

incandescent2  incandescent_spectrum

Fluorescent light

The spectrum of a fluorescent light has bright lines and a continuous spectrum. The bright lines come from mercury gas inside the tube while the continuous spectrum comes from the phosphor coating lining the interior of the tube.

fluorescent fluorescent_spectrum

Sunlight

The solar spectrum is a continuous spectrum of an incandescent gas with some fine dark lines. These fine lines are fraunhofer lines produced by the gases above the surface of the sun which absorbs some of the incandescent light from the sun below.

sun1 sun_spectrum1

Neon light

The spectrum of the neon light has several bright red lines. However, not all neon lights contain necessarily contain neon gas, even though they are called neon lights. Some contain argon or other gasses that produce different spectra.

bluesign bluesign_spectrum

Blue neon sign

neon neon_spectrum

Red neon sign

Computer Display

Spectrum of cathode ray tube on the left and LCD monitor on right.

crt_spectrum laptop_spectrum

Check out some more pictures. Thanks Jerry. Also try this Java based virtual spectrometer.

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