The Cathode Ray Tube

The cathode ray tube is a device used in old cathode ray tube television sets and computer screens. Inside the tube is a vacuum. Electrons are emitted from the heated negative cathode A at the back of the tube and are free to move, not meeting any gas molecules. They are accelerated and focused into a beam first by an anode, cooled so as not to emit electrons by thermionic emission then by magnets at B. Having passed through the focusing magnets at B, they enter an electric field between C and D. The electric field can be changed so the beam is directed towards certain spots on the screen. When they hit the screen, which is coated with a fluorescent material, they emit light of a colour which depends of the applied voltage.

TV and PC screens work by moving the beam of electrons across and down the screen many times each second, The applied voltage is changed so that light of the required colour is emitted from the required spot on the screen. This is done for every spot on the screen many times a second to simulate a moving image.

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