question for smart people, about tv rays and whatnot?

Question

ok so i pluck the string on my bass, and i watch it vibrate, its a sharp fuzzy blur that i see. now when i pluck the string and the tv is behind it in my line of vision, the string has a slower vibration movement and i can see the string waving.
like this pic, if that makes sense http://www.karbosguide.com/books/videosound/sound_files/image003.gif

my question is why can i see this in front of a tv or computer monitor?

Answer 1

Because the light from a TV or monitor is not constant, it is actually flashing on and off faster than you can see. When you pluck your string in front of the TV, the light from the screen illuminates the string off and on, so you only see the string as a series of quick images, first in one position, then another. This makes it appear as if the string is moving slower than it really is.

Answer 2

This is because the TV or monitor does not actually display a steady picture. In reality, these devices (assuming they use a CRT; with LCD screens it will likely not work) actually display only a single dot at any particular time! The reason you see a picture is because it shows so many of these dots in such a quick sequence that your eyes don't notice it.

Your eyes "assemble" the TV picture from these individual dots in exactly the same way that they "assemble" the fuzzy blur from the fast-vibrating string!

Now, if your string is in front of the TV, the string is actually almost invisible because the TV screen is mostly black or dark grey. The string only becomes "visible" because it blocks your view of some of those dots from the TV picture. Since those dots are only displayed for a very very short amount of time, you will only see what the string's position was at that precise moment when the dots in that place were rendered (and your view of those dots was blocked by the string). You don't see the rest of the string's movement because it isn't blocking your view of any other dots.

I think TVs have a refresh rate of about 24 frames per seconds. That means that your eye will only notice the position of the string once in every 1/24th of a second.

Normal TV signals have 720x480 pixels (dots). This means that each dot is only visible for 1/8294400th of a second! By contast, your string is probably vibrating at a kilohertz or two, which means about 1000 or 2000 times per second (it depends on what note you play). Compared to the dots on the screen, the string is really slow!

That is why in front of the TV screen the string will seem motionless for 1/24th of a second.

Answer 3

Because the TV or the monitor is emitting a light that is vibrating at a frequency half the supply frequency. If you are in USA, the supply frequency is 60Hz and the picture on the TV is made up in 1/30th of a second. This flickering light interferes with the string's own vibrations and what you see is an interference pattern of the string. A stroboscope also works on a similar principle.

You can't see that effect with a tungsten incandescent lamp.

Answer 4

The vibrations of the string when viewed in front of the TV are interacting with the frequency of the video lines on the TV.