The vaporized sodium atoms absorb energy from the electric current. They can relax back down to a lower energy state by emitting photons of bright yellow light. The yellow light is extremely intense, and is characteristic of sodium.
You can see the yellow line in the sodium spectrum by looking at the street lamp through a diffraction grating or a spectroscope. Demonstrate that the yellow line is really characteristic of sodium by sprinkling different sodium compounds into a flame. The flame burns bright yellow, and a spectroscope reveals the same wavelength for the yellow line each time (the so-called "D" line). The D line is also very strong in the spectrum of the sun.
The mercury is added to street lamps to introduce some blue and green lines into the spectrum so that colors can be distinguished under the lamp. A pure sodium lamp would produce a nightmarish, monochromatic black-and-yellow effect!
When street lamps get old, one end of the tube becomes much brighter than the other. This is a consequence of sodium's low ionization energy. Sodium atoms become gaseous Na+ ions in the tube and collect on the cathode (negative electrode). The positive end of the tube goes dark because when no sodium remains on that end of the tube.
For more about how gas discharge lamps work, see Goldwaser and Klipstein's detailed technical guide. Here are a few links with more information about atomic emission spectra:
NIST Atomic Spectroscopic Database
Spectra from Space
Energy levels, transition probabilities, and wavelengths for atomic line spectra. Ionization energies for neutral atoms are also available on a hyperlinked periodic table.
A beautifully illustrated K-12 level exploration of four NASA spacecraft with instruments that collect gamma ray, X-ray, UV/visible, and infrared spectra. The site includes hands-on activities including the simulation of IR detection using liquid crystal sheets, studies of UV protection using UV-sensitive plastic beads, instructions for building a simple spectroscope, and even gamma ray and X-ray activities (simulated, of course!)
Graphic showing qualitative line spectra for common discharge lamps.
Author: Fred Senese firstname.lastname@example.org
Copyright © 1997-2010 by Fred Senese
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/electrons/faq/print-orange-streetlights.shtml