A tube light consists of a glass tube containing two filament electrodes, a coating of activated phosphor and small amount of mercury. The two electrodes are placed at the two ends of the tube light. They emit sufficient electrons to initiate electronic discharge in the tube light.
This electric discharge occurs, due to the interaction of electrons with that of the atoms of the mercury. This discharge is responsible for the growing of the tube light. The emission of electrons from the electrodes is mainly in the form of streams that move across the length of tube, but a small number of electrons are also emitted around the tube.
Whenever the tube light is switched on, a comparatively small number of electrons around the electrodes strike the coated walls directly and gradually uncoat the surface. Therefore, after prolonged use, the ends of the tube light get blackened.