television on television

[benji]

why do computer and television screens appear flickery when filmed? also why do vehicle wheels appear to be going backwards when seen on film/tv?

[LeMarchand]

Someone can probably give a more detailed explanation, but I seem to remember that the "screen" effect is caused because both the picture on screen and the film you are watching are composed of lots of still images that change so fast that you see them as one moving image. If one set changes faster than the other (can't remember which way) you get a flicker.

I would imagine that the wheel effect has a similar root cause.

[Chillum]

re. wheels - it's called the "wagon-wheel effect", many explanations availabe on web, here's one  http://www.newton.dep.anl.gov/askasci/gen01/gen01404.htm