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Question

Why is it that a magnet distorts the image from a television or computer monitor (even though photons are uncharged)?
Asked by: Mark Jorgensen

Answer

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. Let's start by looking at the red signal.

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.
Answered by: Jules Seeley, M.S., Physics graduate; Strategy Consultant, London.



Science Quote

'Our job in physics is to see things simply, to understand a great many complicated phenomena, in terms of a few simple principles.'

Steven Weinberg
(1933-)


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