When a television receives a signal, it first splits off the audio (sound) signal and the picture signal from a carrier wave (which is used to allow the signal to be transmitted over long distances). The audio is sent straight to the speakers to produce sound. The picture signal consists of three elements, red, green and blue. A standard television has three ‘electron guns’ at the back of the set, one for each colour. The red signal is fed into one of these ‘guns’. The gun produces a beam of electrons that varies in intensity with the strength of the red signal. This beam is fired towards the tv screen. The electron beam starts at the top-left of the screen and magnetic fields are used to ‘sweep’ this beam across the screen in parallel horizontal lines (if you look closely at a tv screen you can see these lines). UK televisions (PAL) have 625 lines and update the picture 25 times per second, US televisions (NTSC) have 525 lines but update 30 times per second. The back of the tv screen is covered in phosphor ‘dots’ (pixels) which glow when they are struck by these electrons. The red-signal electron beam is aimed so that it strikes phosphor dots that glow red, emitting photons which the eye can detect. The same process occurs for green and blue; each colour signal goes to one particular electron gun which excites just the dots of that colour, the signal tells the gun how strong it should be which in turn means some dots glow brighter than others. When you sit back from the tv screen, you don’t notice the dots nor the flicker, your eye blends the image together to give a clear picture which appears to move. Now to answer the question! A magnet distorts the picture as it distorts the path of electrons flowing from the electron gun towards the screen inside the tv. As electrons are negatively charged particles, their motion is distorted by a magnet. So it is these electrons, not photons, which are distorted by the magnet. On older tvs, damage caused by holding a magnet too close to a tv could be permanent; newer tvs tend to have a demagnetisation process when you switch them on, to ensure that the picture is not permanently distorted.
– The picture on an old fashioned TV is formed by a Cathode Ray Tube, often called a CRT. It uses an electron beam, deflected by magnets, to “paint” the picture. When an electron in a vacuum moves through a magnetic field, the field bends the path of the electron. That’s how the picture is drawn in the CRT. The screen is phosphorescent, and when it hits the different colored dots on the front of the screen, they glow in red, green, or blue if it is a color TV, or just white if it is a black and white TV. Anyway, if you put a magnet near the front of the TV, or perhaps the back or side, depending on the strength, it will deflect the electron beam from the intended path. In the case of a color TV, it will also cause weird colors, because the electron beam isn’t hitting the color of dot that it is supposed to hit. When you remove the magnet, you may see some weird colors persisting. As you turn on and off the tv, a degaussing magnet will eventually remove any residual magnetism. Once I was working in an office that had a radio antenna just outside the wall of my office, just feet away. It was well grounded, to prevent fire from a lightning strike. But it did attract lightning. I know this because when I was working during a thunderstorm, lightning struck the tower. It was a very loud noise to have lightning strike maybe 10 feet away! Anyway, not only did I “levitate” a few feet out of my chair from reflex at my fright from the noise, my monitor appeared ruined. All the colors were way off. Moving electric charge produces magnetism, and in lightning there is lots of moving charge! So my CRT was magnetized, throwing off all the colors. However, I turned my monitor on and off about 20 times, and all the colors were returned to normal by the degaussing coil, which is at the front of the CRT. Thankfully, because of good wiring, the lightning didn’t get into the power line, and none of the equipment at the office where I worked was damaged.