Different light bulbs create their light in different ways, ways you can examine with a diffraction grating.
If it weren't for the diffraction grating, it's likely no one would know that matter is made of atoms. The wavelength of light corresponds to the energy of the source of the light. A diffraction grating separates light into its separate wavelengths, so putting light through a diffraction grating provides a direct measurement of the energy in the source that created the light. This effect gave scientists a tool to examine the energy structure of matter. A spectrometer is an instrument that uses a prism or, more commonly, a diffraction grating, to direct light of certain wavelengths to specific locations in a detector.
How Diffraction Works
When two light beams from the same source are split from each other and then brought back together, they interfere with each other. This means that the electric field adds or subtracts to create an illumination structure that has a bright and dark pattern, instead of the uniform blob of light you'd see if the source wasn't split. A diffraction grating splits a light beam into hundreds or thousands of separate sources. Those sources interfere. Since the interference depends on wavelength, where yellow light, for example, adds to make a bright spot, the other colors will subtract to make a dark spot --- so only yellow is seen there. Each color forms a bright spot where the other colors are not seen.
Building a Spectrometer
A compact disc is a diffraction grating. Each of the narrow grooves reflects a different part of the beam, splitting one beam into many beams, then recombining them. You can make your own spectrometer with just a pie-shaped wedge cut from a compact disc. You can see the principle if you hold the wedge so you see the main reflection of a light bulb in the shiny surface, then slowly tilt the wedge toward you. You'll see a stretched-out reflection of the bulb, stretched out in different colors. You can build your own spectrometer with a CD and a cardboard box. Several plans are available on the Internet. The spectrometer will make it easier to keep the image stable if you decide to take pictures of your spectra.
Examining Different Sources
Different illumination sources produce light in different ways. With a CD diffraction grating or your homemade spectrometer, you can see the effects of their different mechanisms. Look at the spectrum of an incandescent bulb and you'll see a continuous band of light, from red to indigo. Try looking at the spectrum of a fluorescent bulb. You'll notice several bright colors, separated by darkness. You can also look at different streetlights and see what different colors combine to make their light. If you want to look at the spectrum of the sun, do not look directly at your diffraction grating. Instead, look at the light projected on a piece of white paper.
Looking at Absorption
The spectra you saw in the experiments of the last section are the result of atoms converting energy in their electrons into light. When the process goes the other way, with atoms absorbing light and adding the energy to their electrons, instead of showing up as lines of light, the process is seen as bands of darkness. You can send a bright light source in your spectrometer, then place a clear container --- called a "cuvette" --- in the path of the light. Look for dark bands in the spectrum, showing where your sample absorbs light.
Commercial Spectrometers
Hand-sized commercial spectrometers are now available. These devices include a diffraction grating and a detector in a convenient housing, along with electronics and a connector that will let you send the data directly to your computer. These devices make it possible to make quantitative measurements of spectra. For example, you could measure the difference in the spectra of a 100-watt incandescent bulb and a 15-watt refrigerator bulb -- or the difference between the exhaust from your car and your motorcycle.
Tags: diffraction grating, your spectrometer, beam into, bright spot, compact disc
