You often tend to see sodium vapor lamps along the roads and parking spaces. The basic principle behind the working of sodium vapor lamp is passing electric current through sodium gas to make the gas emit light. This is called photoelectric effect. There is a U-shaped arc tube made of glass. For an easy start up, small amounts of neon and argon gases are also present along with sodium gas inside the glass.
Passing current through the gas increases its temperature and the gas vaporizes. A sodium atom has 11 electrons, and because of the way they're stacked in orbit, one of the electrons is most likely to accept and emit energy. There is an electric discharge when passing electricity through sodium vapors at high or low pressure. This causes the valence electron of the sodium atoms to excite to higher energy levels. The excited electrons emit light around a wavelength of 590 nanometers. This wavelength corresponds to yellow light.
As they produce only yellow light, they cannot be used for ordinary lighting purposes though their output is very high. This is because most of the light emitted from a sodium vapor lamp is concentrated in the yellow part of the visible spectrum.
Mercury-vapor or fluorescent lamps are also used for most street lighting. These lamps consume less electrical power than incandescent lamps to produce the same amount of light. Sodium vapor lamps are very effective in fog while fluorescent lamps produce little glare. Sodium vapor lamp is the most energy efficient of all other lamps because it uses all its current to produce yellow light. Due to the long life, efficient energy usage and good vision, sodium vapor lamps are used for street lighting.