Fig. 1: One of the designs for a do-it-yourself visible light spectrometer by Public Lab.
The way chemicals and elements absorb or emit light provides a powerful method for analyzing the chemical composition of a sample, and one that people can do surprisingly cheaply. The basic principle of spectrometry is that different molecules or atoms absorb or emit light in different wavelengths. Therefore, one can get a great deal of information about a sample by looking at which wavelengths of light get absorbed as they pass through it. Alternatively, if the sample emits light (e.g. a flame, a star, or a fluorescent molecule) one can get information based on the wavelengths of light emitted.
A spectrometer measures the intensity of light across a range of wavelengths to generate an absorption or emission spectrum of the sample under investigation. Although spectrometers can be quite pricey, and cover a wide range of from x-ray to radio frequencies, home built visible light spectroscopes can be provide a great deal of information. In fact, several chemical elements were discovered using visible light spectroscopy in the 19th century (e.g. helium, cesium, and rubidium). These early techniques required little more than a light source (e.g. a flame or the sun) and a prism. Cheap digital cameras and diffraction gratings made from CD's and DVD's make it possible for anyone to cheaply build a simple visible light spectrometer. Examples include two designs by Public Lab that include software for quantitatively analyzing and comparing spectra.