23.5Dispersive spectroscopy

Some dispersive analyzers use a reflection grating instead of a refraction grating. Reflection gratings use fine lines etched on a reflective (mirror) surface to produce an equivalent dispersive e ect to a di raction grating40:

Violet

Blue

Green

Yellow

Orange

Red

In 1814, the German physicist Joseph von Fraunhofer closely analyzed the spectrum of colors obtained from sunlight and noticed the existence of several dark bands in the otherwise uninterrupted spectrum where specific colors seemed to be attenuated. Later that century, experiments by the French physicist Jean Bernard L´eon Foucault and the German physicist Gustav Robert Kircho confirmed the same e ect when white light was passed through a vapor of sodium. They correctly reasoned that the sun’s core produced a continuous spectrum41 of light (all wavelengths) due to its

40You may use an old compact disk (CD) as a simple reflection and refraction grating. Holding the CD with the reflective (shiny) surface angled toward you, light reflected from a bright source such as a lamp (avoid using the sun, as you can easily damage your eyes viewing reflected sunlight!) will split into its constituent colors by reflection o the CD’s surface. Lines in the plastic of the CD perform the dispersion of wavelengths. You will likely have to experiment with the angle you hold the CD, pointing it more perpendicular to the lamp’s direction and more angled to your eyes, before you see the image of the lamp “smeared” as a colorful spectrum. To use the CD as a di raction grating, you will have to carefully peel the reflective aluminum foil o the front side of the disk. Use a sharp tool to scribe the disk’s front surface from center to outer edge (tracing a radius line), then use sticky tape to carefully peel the scribed foil o the plastic disk. When you are finished removing all the foil, you may look through the transparent plastic and see spectra from light sources on the other side. Once again, experimentation is in order to find the optimum viewing angle, and be sure to avoid looking at the sun this way!

41One might wonder why the sun does not produce a line-type emission spectrum of all its constituent elements, instead of the continuous spectrum it does. The answer to this question is that emission spectra are produced only when the “excited” atoms are in relative isolation from each other, such as is the case in a low-pressure gas. In solids, liquids, and high-pressure gases, the close proximity of the atoms to each other creates many di erent opportunities for electrons to “jump” to lower energy levels. With all those di erent alternatives, the electrons emit a whole range of di erent wavelength photons as they seek lower energy levels, not just the few wavelengths associated with the limited energy levels o ered by an isolated atom. We see the same e ect on Earth when we heat metals: the electrons in a solid or liquid metal sample have so many di erent optional energy levels to “fall” to, they end up emitting a broad spectrum of wavelengths instead of just a few. In this way, a molten metal is a good approximation of a blackbody photon source.