The Picture Tube

In the picture tube, electrons are accelerated from the cathode to the screen where they strike the phosphorous coating. The kinetic energy of the electrons is transferred to the phosphor which glows by emitting white light. By making the electron beam scan the screen, dots of light glow, creating an image which we perceive. Of course the individual dots must light up fast enough in succession so that we get the illusion of a one continuous picture. Indeed the beam starts at the upper left hand corner of the screen and moves to the right. Once a line the beam can move to the next line and so on until it reaches the bottom of the screen. Such a procedure would take 1/30th of a second which is a bit slow as our eyes will perceive the flicker. The trick which is actually adopted is to first scan all the odd number lines and then go back to the top to scan the even numbered lines. In this way a picture frame is formed every 1/60th of a second which is sufficient for smooth viewing. Electric and magnetic fields within the tube are used for focusing the beam. The horizontal and vertical deflection of the electron beam is performed by magnets using the Lorentz force discussed earlier: A vertical magnetic field deflects the electrons horizontally while a horizontal magnetic field deflects the electrons vertically. Shades of gray on the screen are produced by controlling the amount of current in the beam. This is done by changing the amount of negative charge on the grid in front of the cathode: Thus effectively electrons are repelled by the grid and so not all of them make it to the anode and then on to the screen. A colour picture tube has three separate electron guns and a screen coated with phosphor dots of the three primary colours: Red, Green and Blue. Between the beams and the screen are three masks with holes aligned so that each beam can only strike dots of one colour.