The Smithsonian Institution’s National Museum of American History, on the national mall in Washington, D.C., has an exhibit on Thomas Edison that includes the development of electricity and lighting, and, more generally, invention (http://americanhistory.si.edu/lighting/). One area that I saw in late 2017 had a set of lamps, including a low-pressure sodium vapor lamp, a mercury lamp, an incandescent lamp, and a compact fluorescent lamp. Here is a close-up of part of the exhibit:
As shown by the captions under each lamp in the picture, the emphasis of the exhibit is on energy efficiency. The low-pressure sodium vapor lamp puts out the same amount of light as the mercury vapor lamp or incandescent lamp but at lower power. However, the first thing a spectroscopist notices is that the perceived color of each lamp is different. My instinct was to look at the exhibit through a diffraction grating. And, thanks to having a SpectroBurst™ Viewer at hand, I had one! In fact, I had two, but I chose the 500 line per millimeter linear grating (blue frame in the Viewer set). While I had located incandescent lamps, fluorescents, and high-pressure sodium vapor lamps around our neighborhood in Champaign, I had yet to find a low-pressure sodium vapor lamp to observe for comparison with its high-pressure cousin. Here was my chance! Low-pressure sodium vapor lamps emit the sodium doublet at 588.9 nm and 589.5 nm rather than a broad continuum as comes from high pressure sodium vapor lamps. What do the spectra look like?
First, here’s a picture of your intrepid blogger taking a picture without a grating.
Second, I’ll show the unannotated picture of the exhibit through the grating. Then I’ll mark it up and describe what is seen.
This looks messy, because there are many spectra generated from the three different lamps. There are multiple images of the sodium vapor lamp. The mercury lamp puts out spectra to left and right (since the camera was mostly centered on the mercury lamp). From the right, there are continua from the incandescent lamp. Around the edges are spectra whose source and identity aren’t immediately obvious.
So I marked off all the lines I could identify. Here they are:
The yellow lines relate to the low-pressure sodium vapor lamp, the light blue bands identify light from the incandescent lamp, and the green lines relate to the mercury lamp. There are additional continua from the incandescent lamp to the right of the field of view. Let’s dissect these spectra.
Low-pressure sodium vapor lamp (identified with yellow lines)
At the left, middle, is zero-order for the sodium vapor lamp. Because the lamp is large, the 588.9 nm/589.5 nm doublet cannot be resolved. From the center of the zero order image to the center of the first order image is about 3/8 of the picture width, and second order is another 3/8 of the way across. As one would expect, the space between orders is about the same. Where’s third order? It’s off to the right of the field of view.
Near the bottom of the picture, there’s a reflection of the light from the lamp. And sure enough, first and (weakly) second order show up as the grating disperses light from the reflection as well as from the original lamp.
The diffraction orders are tilted to make for easier annotation (I was thinking ahead). If I had rotated the grating about 5° clockwise, the orders would have been dispersed parallel to the horizontal edges of the photo, but then distinguishing some orders from each other would have been more difficult.
Incandescent lamp (identified with blue lines)
Zero order shows a highly saturated image of the incandescent lamp. First order (with the visible range marked in the image) comes not only from the filament but also from glints off the glass envelope of the bulb. There is a weak second order spectrum farther to the left but I left it unmarked because the mercury arc lamp and sodium vapor lamp give more prominent features in this region. But can you see the continuous spectrum there?
Mercury arc lamp (identified with green lines)
Analyzing these lines is pretty simple at f