| From: Josh | 16/09/99
11:08:18 |
| Subject: spinning wheels | post id:
38076 |
| Why is it that if you watch a
car, bike, etc, it sometimes looks like the wheels are slowly spinning
backwards? | |
| From: Michael Gunter | 16/09/99
11:20:55 |
| Subject: re: spinning wheels | post id:
38082 |
A couple of
possibilities:
| |
| From: lEANNE wOODWARD | 16/09/99
11:25:28 |
| Subject: re: spinning wheels | post id:
38093 |
| The wheels look as if they spin
the other way, because of optical illusions. these are created by the
interpretation your brain makes about what the eyes see. the eyes can
actually only interpret a certain amount of frames a second. any more and
the messages aren't noticed by the conscious brain. A wheel may travel at
speeds, at which your eyes capture frames, like a movie or camera, (frames
being missed in the interim). If your eyes catch one frame every
revolution that the wheel makes, then the wheel may not appear to be
moving, if your brain catches more than one frame for every wheel
revolution then the wheel will seem to be moving forward until a point. As
the wheel gets faster, the eye can catch less pictures of the wheel for
every revolution. If you catch onen frame and just another, before it
reaches a full turn, then the second frame you see has actually gone back
wards, and so on, to make the wheel appear to be moving in the other
direction. This is much like what happens in a disco, with the flasking
lights. As you watch some one dance while these lights are on, your brain
is able to see the effect of only capturing certain frames, of what is
happening. The dancers seem to move from one position to another with
nothing in between, like what is happening with the wheel. You don't get
to see the majority of the wheel movement.
| |
| From: askegg | 16/09/99
11:29:37 |
| Subject: re: spinning wheels | post id:
38096 |
| Your eyes sample the outside
world at a given rate (can't remember the number) say 30 times a second
(or 20Hz). Let's say the wheel has 4 spokes (makes it easy to do the sums) - if the wheel is spinning fast enough to make a quarter turn 20 times a second (or 5/sec, 300rpm) the wheel does not appear to turn because your eyes only ever see it in the same position. If you speed the wheel up just a little the wheel will appear to move slowly because everytime your eyes sample the wheel it appears to have moved a little, even though it may have actually moved 5/16 of a circle. Basically - it's all to do with the frequency of your eyes and what your looking at .... | |
| From: Michael Gunter | 16/09/99
12:22:27 |
| Subject: re: spinning wheels | post id:
38184 |
| I still reckon the eyeballs are
made of jelly and they (sometimes) jiggle, like other bits of human
anatomy! To prove this, go outside across the yard, look through a window at the TV, and see how the picture behaves when you eat some crunchy food. | |
| From: Halogen Fisk | 16/09/99
12:35:22 |
| Subject: re: spinning wheels | post id:
38192 |
| There has been much confusion on
this site RE: Eye's sampling rate. Many people attributed stobing effects to thte "flicker Flicker fusion is what merges strobed images to give the illusion
of The "Flicker Fusion Rate" is the
frequency The eye DOES NOT SAMPLE AT A FRAME RATE. Strobe effects (such as wheels turning backwards) are normally
seen | |
| From: Michael Bowden | 16/09/99
12:39:46 |
| Subject: re: spinning wheels | post id:
38194 |
| Not sure what this has to do with
spinning wheels, but I have an explanation for the way a TV screen shakes
and flickers when you eat crunchy food. The image on a TV screen is refreshed 50 times every second. This is not fast. My computer monitor is refreshing 100 times a second right now, but I can go to 160 times a second (Hertz) with ease. If you hold your open hand up between your eyes and the screen and shake your hand so your fingers flick quickly from side to side, you will also see the kind of flicker you get when you eat crunchy food. Basically, the information you get from the screen is not as constant as you get with real objects in ordinary light. Everything in reality is updated constantly by you visual system, but the TV screen does not update continuously. Removing some of the information such as by flicking your fingers, or disjointing the refresh by changing you eye position slightly (shaking your eyes by eating something) causes the screen to flicker because the flow of information is disjointed partly. Obviously, this does not happens as much on my monitor. Incidently, most old monitors operate at 60Hz, but because of the detail and the proximity to the screen you can get headaches. You may not see the flicker consciously, but it is still there, and your brain doesn't like it. It's like a constant strobe. That's why I got such a good monitor...no headaches. Mr. Whippy | |
| From: James Pitt | 19/09/99
17:20:14 |
| Subject: re: spinning wheels | post id:
38756 |
| I aggree that this illusiont is
caused by a stobe effect. however I belive the source is NOT from
jiggleing eyes or sampling rates of the brain etc , but rather the street
lights at night flickering as a result of the A.C power they run off -,
which put simply, swiches on and of 50 times per second. This effect is
actually used to set the speed of record players accuratly by using whats
called a stobe disc. the disc sits on the turntable and has white and
black stripes running from the center to the outer edge of the disc. the markings are spaced in such a way that if you have a flouresent light running off mains power (50Hz) above the turntable , the disc will give the illusion that it is still when when it is rotating at the correct speed.if the turntable is runnig slightly out of speed the disc will appear to be moving very slowly (possibly even backwards depending on whether the player is running "fast or "slow") . A mag wheel on a car has a similar patern as the stobe disc (shinny spokes with a dark backround) therfore causing a simalar effect . should this theory be correct the illusion would not occor during the day | |