why is it that when you put a magnet on a computer moniter or TV screen it leaves a green spot behind?

Answer 1

The image is created by squirting three streams of electrons down the length of the CRT, though a series of holes near the front and onto the phosphors on the inside of the CRT glass.

The purpose of the little holes (The shadow mask) is to make sure that the electrons in the read electron beam only hit red phosphor dots, the blue beam hits blue phosphor dots and the green beam hits green. The set up and alignment of the shadow mask and the phosphor dots needs to be very precise.

The best material for the shadow mask is iron. Unfortunately you can magnetise that iron with an external magnet. When you shoot electrons though a magnetic field they drift sideways, so now each of those three electron beams are not hitting their respective phosphor dots properly. If the magnetic field is strong enough they can actually hit the wrong phosphor dots.

And that is what you are seeing.

Power the TV / monitor on a few times (waiting 20 minutes between power cycles) and the problem should go away.



LCDs produce their images via electric fields and are so unaffected by magnets.

Answer 2

if you are using a CRT than the answer is very simple. The screen is made up of pixels. each dot is a pixel. these pixels are magnetic and hence get attracted to your magnet.