Sodium lamp is used for street lighting. It is also used in research laboratories as a monochromatic source of light, as it produces bright yellow light which is quite pleasant to the eyes. Do you know how does this lamp work?
A sodium lamp is operated on alternating current. It consists of a U-shaped glass tube with two electrodes of tungsten spiral coated with barium oxide. The tube is evacuated and neon gas at low pressure of about 10 mm of mercury is filled along with a small quantity of metallic sodium or sodium vapour. This discharge tube is enclosed in an unsilvered vacuum jacket to avoid heat loss. For electric discharge a voltage of 400 volts is applied to the electrodes with the help of a transformer. Initially neon gas gets discharged and red light is produced, due to this sodium atoms get excited and produce yellow light. Because the ionization potential of sodium is higher than neon gas, the lamp produces more of sodium light.
The working temperature of the lamp is about 250 degree centigrade. If this temperature is not maintained constantly the intensity of emitted light would be considerably varied. The sodium light contains only two wavelengths, viz 5890°A and 5896°A. Sodium lamp is also used for outdoor illumination as the characteristic yellow light is less absorbed by fog and mist than white lamp.
