# Why does your hand look funny when .....

ivanhoe
Posers and Puzzles 13 Sep '04 23:52
1. 13 Sep '04 23:52

Why does your hand look funny when you move it fast in front of the tv or computer screen ?
2. 14 Sep '04 01:26
probably a stroboscopic effect due to the fact that crt screens aren't a continual light source, but refresh the image several times a second (25 a sec for tv's) from top to bottom
3. 14 Sep '04 07:19
It's not important wether you do it in front of a screen or not. It's just that our eyes take pictures of the world and they do that 10 times a second. So what you see is a series of foto's, each taken 0.1 second after the previous. When things move fast and unsuspected (necessary) you will see a strange series of pictures. We'll think that our hand has got more fingers than usual.
4. 14 Sep '04 07:21
Why does it need to be unsuspected?

For example a fast car, that is just driving around. You will be able to predict where the car will be the next 0.1 second and your mind will not be confused when the car has 'suddenly' moved.
5. 14 Sep '04 09:28
I think it is definitely the stroboscopic effect. If you move your hand fast in just the air, it will blur. But in front of a screen you will see distinct pictures due to the flasing of the screen.
6. 14 Sep '04 09:49
Originally posted by piderman
I think it is definitely the stroboscopic effect. If you move your hand fast in just the air, it will blur. But in front of a screen you will see distinct pictures due to the flasing of the screen.
You are right! There is a difference, I first didn't notice.
But wasn't my explanation right?

In front of a screen, you are right about the stroboscopic effect
7. 17 Sep '04 14:09
Originally posted by sfen
You are right! There is a difference, I first didn't notice.
But wasn't my explanation right?

In front of a screen, you are right about the stroboscopic effect
Yes, sometimes we see six fingers instead of five when we move our hand in front of tv screen with fingers spread apart. This happens due to the combined effect of the following two phenomena.
(a) TV Screen flickering @ 24 pictures per second with darkness
in between.
(b) Persistence of vision of our own visual system, which lasts for
approximately 1/10 seconds.
By the time one sequence of brightness is replaced , after the brief spell of darkness , by the next sequence of brightness, the fingers have moved to a new position. The first finger thus appears to the left( assuming the movement was towards left) of its earlier position and so on for each of the five fingers. But at the rightmost finger's older position,where it is no longer there , you still see a finger due to persistence of vision. Thus in all you see six fingers.
If the speed of the movement is very fast ,sometimes you can see seven fingers, due to this same effect.
8. 17 Sep '04 14:14
Originally posted by piderman
I think it is definitely the stroboscopic effect. If you move your hand fast in just the air, it will blur. But in front of a screen you will see distinct pictures due to the flasing of the screen.
What the hell is this STROBOSCOPIC EFFECT ?
I cant make the head or tail of it.
9. Acolyte
17 Sep '04 23:051 edit
Originally posted by observantU
What the hell is this STROBOSCOPIC EFFECT ?
I cant make the head or tail of it.
It's to do with how our eyes work. When light hits the eyes, the receptors remain stimulated for a few milliseconds after the source of light is removed - the greater the exposure, the longer the after-image. Dazzling is an extreme case of this.

The stroboscopic effect is when you have an intermittent, periodic source of bright light. The illuminated after-images last much longer than the non-illuminated ones, so you effectively don't see the dark bits at all. The result is that each flash appears to 'freeze' everything until the next flash. This can be quite useful - for example, if you shine a strobe at an engine and adjust the frequency of the strobe, the engine will appear to stop when the two are in sync, giving you the frequency of its rotation. Move it slightly out of sync and you can see how it works, only in slow motion.
10. 18 Sep '04 00:13
Originally posted by ivanhoe

Why does your hand look funny when you move it fast in front of the tv or computer screen ?

Ivan, just why is your hand moving fast infront of your screen ? ðŸ˜‰

11. 18 Sep '04 01:17
Originally posted by pcaspian
Originally posted by ivanhoe
[b]
Why does your hand look funny when you move it fast in front of the tv or computer screen ?

Ivan, just why is your hand moving fast infront of your screen ? ðŸ˜‰

[/b]

I was chasing away some flies ...... ðŸ˜‰

12. 18 Sep '04 09:26
Originally posted by Acolyte
It's to do with how our eyes work. When light hits the eyes, the receptors remain stimulated for a few milliseconds after the source of light is removed - the greater the exposure, the longer the after-image. Dazzling is an extreme case of this.

The stroboscopic effect is when you have an intermittent, periodic source of bright light. The illuminated ...[text shortened]... f its rotation. Move it slightly out of sync and you can see how it works, only in slow motion.
Yes - Your eyes also need a bit of blur to make proper sense of movement. That is why stop frame animation looks jerky - even though the update rate is the same as normal tv there is no blur, so the movement looks strange. Its interesting that this means we must see moving things blurred in real life, because they don't look jerky.