Street Lamp Spectra

What it shows:

Unlike the continuous spectrum emitted by blackbody radiators, the light given off by atoms in a gaseous discharge is characterized by its discreet nature. Using street lamps for the light sources, bright atomic spectra of mercury or sodium are projected onto a screen.


How it works:

Either Hg or Na street lamps can be used for this demonstration, but Hg has by far a more interesting (richer) spectrum and will be used as the example in this description. A 1000 watt lamp (the type used to illuminate streets, parking lots, etc.) serves as the brilliant source of light. 1 A special reflective diffraction grating images the spectrum onto a screen. It is a concave, front-surface mirror with 540 grooves per mm (≈13,700 per inch) engraved on its surface. 2

Four images of the lamp fill the screen. The red image will seem quite blurred, or out of focus. This is because the red is actually due to the fluorescence of the coating on the glass and is not a Hg line -- it consists of a range of red colors emitted by the coating. The yellow image also appears slightly blurred to the audience although, up close, it is obvious that it actually is two separate but overlapping yellow images of the lamp. These two images correspond to the Hg yellow doublet (5789.7 and 5769.6 Å) and the two to three centimeter separation on the screen corresponds to the 20 Å difference in wavelengths. The bright green image is the 5460.7 Å line while the blue image is at 4358.4 Å.

If it is desired to show the ultraviolet Hg lines, we have a second lamp whose thick glass envelope has been removed for that purpose. The image of the inner quartz discharge tube, being tubular in shape, is more akin to a line spectrum. Indeed, the yellow doublet is easily resolved as two separate images and the red is, of course, gone since it was not due to the Hg. But what is now present is a plethora of UV lines which can easily be made visible by holding up a new white T-shirt (which fluoresces marvelously). A special sensitive UV screen is also available and we have counted as many as 31 UV lines (which appear green on the screen). The 1st UV line (4046.6 Å) is quite close to the visible. [CAUTION: Wear safety goggles to protect the eyes against the intense UV, particularly when you have to go near the lamp. Furthermore, the UV produces a lot of ozone which is poisonous -- avoid breathing the ozone. Finally, high voltage electrodes are exposed with the glass envelope off -- do not touch. Put up signs on all doors leading to the lamp warning against INTENSE UV, OZONE, and HIGH VOLTAGE]

Setting it up:

The long focal length of the mirror (5.343 meters) limits its use to lecture halls B and C. The lamp (placed on a small cart) sits outside the lecture hall -- the doors are cracked open only enough to illuminate the grating, which also sits on a small (but tall) cart on the opposite side of the lecture hall, about 13 meters from the mirror. Given the focal length, the screen should be 9 meters from the mirror, 3 which puts it on the opposite wall close to the door. The image will be just a little bit smaller than life-size and, of course, upside down. Both the Hg and Na lamps take about 10 minutes to warm up to full brightness. This should be kept in mind by the lecturer and planned for accordingly. Also note that, once turned off, the lamp must cool down before it can be turned on again.


For lecture halls A, D, and E, use the smaller (400 watt) Hg and/or Na lamps. These are mounted in elliptic reflectors and use the in-line prism for dispersion. They are described in detail in Planck's Constant and Collisional Broadening.


Big is always dramatic, and this spectrum is big! Furthermore, the advantage of projecting a spectrum on a large screen is that the lecturer can literally walk through it and point out features being discussed. (When individual audience members are peering through their own hand-held diffraction gratings, one can never be sure that they're looking at the right feature, or even looking through the grating the right way!) Lastly, the monochromatic images of the street lamp are a good reminder that the lines one normally sees in a line spectrum are, in fact, individual monochromatic images of the slit in that particular optical setup. Rating ***

1 Philips Mercury Vapor High Intensity Discharge (HID) Bulb; model H36GW-1000DX. The bulb is rated at 63,000 lumens and has a 24,000 hour life (available for $47 in 1988). The bulb fits into a Lumarck General Purpose Fixture, model MHss-1000-MT, type GWA - a 1 kW ballast transformer with a mogul base socket ($153 in 1988).
2 Baush and Lomb cat. no. The 9" diameter mirror has a 10685 mm radius and the grating is blazed at a 4°39' angle. The grating is on loan to us from the G. Harrison Spectroscopy Lab at MIT.
3 Remember that the inverse of the focal length is equal to the sum of the inverses of the object and image distances.