The contents of an ordinary tube light used for lighting purposes are which of the following?

Options :
  1. fluorescent material and an inert gas
  2. one filament, reflective material and mercury vapour
  3. fluorescent material and mercury vapour
  4. two filaments, fluorescent material and mercury vapour
Answer and Explanation :-

Answer: Option 4


An ordinary tube light or a fluorescent lamp or fluorescent tube is a low pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. A fluorescent lamp basically consists of a long glass gas discharge tube. Its inner surface is coated with phosphorus and is filled with an inert gas, generally argon, with a trace of mercury. The tube is then finally sealed at low pressure with two filament electrodes each at its both ends. These electrode filaments are used to preheat the tube and initiate a rapid conduction of electrons between the two end electrodes. The process initially requires a relatively high amount of power. The energy also converts some of the mercury from liquid to vapour. Electrons then collide with the gaseous mercury atoms, increasing the amount of energy. As electrons return to their original energy level, they begin to release light. However, the light they emit is ultraviolet, and not visible to the naked eye, so another step needs to take place before light can be seen. This is why the tube is coated with phosphorus. Phosphorus gives off light when exposed to light. When exposed to the ultraviolet light, the particles emit a white light which we can see. Once the conduction of electrons between the electrodes is complete, no more heating of the filaments is required and whole system works at a much lower current.

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