The constituent colors in a beam of light are revealed when the light is dispersed by a raindrop or passed through optical instruments known as prisms and diffraction gratings. In each case, an array of colors, or spectrum, is observed.Much of what we know about the makeup of matter has been gained through spectroscopy, the study of spectra. Our current understanding of atomic structure can be traced back to Johann Balmer's analysis of the spectrum of hydrogen. On a larger scale, just about everything we know about objects in the far reaches of the universe comes from examining light. Since each element emits a unique collection of colors, spectra, like fingerprints, may be used to determine the makeup and physical condition of stars.The colors contained in light from a particular source are revealed when the light is scattered from an optical instrument called a diffraction grating. Unlike a prism, which relies on refraction to disperse different wavelengths of light, a diffraction grating employs interference to separate spectral colors. A grating consists of a very large number of equally spaced parallel lines etched on the surface of either glass, plastic or metal.
While most people are familiar with the beautiful array of colors reflected from the surface of a compact disc, they may not realize that, like a diffraction grating, a compact disc is covered with thousands of tightly-spaced, nearly parallel lines. These lines are sections of a spiraling line engraved on the disc's surface. This data spiral contains the information-bearing pits that are burned on the disc during the recording process.The availability and low cost of recordable compact discs make it possible for every student in your class to make and use their own spectroscope. In this edition of CoolStuff we provide instructions for making three types of CD-R spectroscopes. The inexpensive devices are easy to construct and will allow students to study the spectra of light sources both in and outside the classroom.
Each type of spectroscope has unique advantages. The reflection spectroscope allows students to observe both emission and absorption spectra. Using this device they can observe the portions of the white light spectrum absorbed by a variety of substances, including chlorophyll. They may be surprised to learn that a leaf, the universal embodiment of greenness, rejects green light and absorbs light from the other portions of the visible spectrum.
Using the entire surface of a clear CD-R disc, the simple transmission spectroscope described below has great light gathering power. It may be used to study the spectra of sources of low intensity such as LEDs and the moon.
The CD-R tube spectroscope resembles many commercially available devices in both appearance and performance. The dramatic spectra produced with this spectroscope are seen throughout this newsletter.
Any one of the three spectroscopes may be used in school or at home. When used in your laboratory, we suggest that you set up as many light sources for observation as possible. These sources may include incandescent bulbs, fluorescent lamps, neon indicators, LEDs, computer monitors…the list goes on. We have included a data sheet that students may use to record their observations. If you have students examine light sources outside the classroom, you may find that they return with spectra of sources such as sodium and mercury streetlights, neon signs, halogen headlights, and electric range elements.