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PHOTO: M. VEDHAN

Answer 1: The commercial form of sodium vapour lamp is a U-shaped tube made of a special kind of glass which does not get blackened by sodium vapour generated during the course of lamp operation.

It has two oxide coated electrodes sealed into it at its ends and connected to the secondary of a step-up transformer whose primary is connected to 220 V 50 hertz AC supply. The tube contains droplets of pure sodium metal uniformly deposited on its inner walls. The tube also contains a very small quantity of neon gas which actually serves as a catalyst to start the discharge.

When the lamp is switched on, an electric discharge starts through the neon vapour. Due to the presence of neon, the colour of light is at first red until the lamp temperature rises to a stage at which sodium gets vapourized. Then the discharge is almost entirely maintained by the sodium vapour when the light turns yellow. It takes some time, say, 5 minutes for the lamp to glow at its full intensity due to the gradual vaporization of sodium.

K. KANNAN

Answer 2: Sodium vapour lamps are basically electric discharge lamps generating visible light through electronic excitation–de excitation processes. As the lamp is switched on, first electric discharge strikes through the mixture of neon-argon gases present in the lamp leading to a pale red glow seen in the initial stage. As the electric discharge through these gases picks up there are inter-ion collisions between these gases resulting in dissipation of energy as heat thereby generating a temperature of about 300 degrees C.

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