Don't tell the Double-Rainbow guy, but all those beautiful colors really can have a deeper meaning. That's because different chemicals reflect and absorb different wavelengths of light, leaving tell-tale patterns in the rainbow. If you know what colors specific chemicals absorb, you can look at the light reflected off a sample and use the rainbow to figure out what you've got. That's basically what a spectrometer does. And blogger Charles Soeder made his own.
The light source is an LED flashlight. The light shines through the sample (in this case a vial of chlorophyll) and gets broken up by a diffraction grating. This produces a spectrum which gets projected onto the photosensor. I pulled the sensor out of an automatic night light. It is mounted on a stand, which is taped to a TI89 which is taped to the table- so I can slide the sensor back and forth along the spectrum to get readings at different frequencies. I measured the frequency of light hitting the detector by noting where its shadow falls on the ruler in the background. The resistance of the sensor changes depending on how much light falls on it (which is an indication of how much light gets absorbed by the sample); I measure this with a multimeter.
There's more photos of all the different parts in his set-up. My favorite features the helpful caption, "This is where the rainbow goes."
(Thanks to Kevin Zelnio!)