Electricity & Megnetism

Image formation on a TV tube

Image formation on a television tube is an important application of the magnetic force acting on a moving electric charge. The image that forms on the television screen, as in the cinema, consists of a series of frames constructed in short intervals of time. It is because of the persistence of the retina of the human eye that the brain continuously interprets images. The image on television is formed by an electronic beam that sweeps across the screen of the image tube. In cinema, the process of image formation occurs in a contrary way to the process that occurs in televisions. The formation of images consists of photographs that are different from each other, which, when moved at high speed, end up forming the image that the human brain captures.

A television tube is made up of the following parts:

• The electronic cannonit is located in the posterior external part to the all. This is a device that emits beams of electrons, which are accelerated through a voltage of thousands of volts;
• A pair of coils that generate a magnetic field both horizontally and vertically. When passing through the coils, the electron beams suffer horizontal and vertical deflection due to the force exerted by the magnetic field;
• The screen is where images are formed. It is covered by a fluorescent material and is hit by the electron beam after being deflected by the magnetic fields.

Upon hitting the screen, the electrons produce a luminous spot at each point they hit. The magnetic fields of the coils, which deflect the electrons, periodically change direction so that the electrons travel at high speed across the screen, from left to right and from top to bottom. If the TV is not tuned to any channel, scanning occurs continuously, resulting in a uniform brightness on the TV screen. When tuning a channel, the signals that are picked up by the antenna modify the sweep, making certain points clearer in relation to others, thus forming the image we see.

The color of the image that appears on the television is determined by the fluorescent material that coats the screen. This material can be black and white as well as green and white, blue and white or red and white. In a color TV, the tube has three electronic cannons, one for each primary color of light (green, blue and red), it’s like three tubes. The screen, in turn, is composed of numerous triple points, phosphorescent, which emit light when hit by electron beams.

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