Hey, I think I may have found a slight error or at least a significantly incomplete answer to the question:

#19: "Why does a wheel seem to move backwards as it speeds up?"

on the BA's 1996 MADSCI Q&A.

http://www.badastronomy.com/mad/1996/wheel.html

He rightly relates the phenomenon to the frame rate of film, but I'm surprised that there was no mention of this effect as seen in real life.

I always thought this was strictly a film-thing myself until I happened to see this a number of times, directly with my own eyes. No film.

Has anyone else seen this too? I'm sure it exists. And I would guess it is somehow caused by the number of spokes on the wheel in relation to the number of revolutions / minute.

Can anyone help me out here?

RBG

yes. bear in mind that I'm no good at the code necessary to insert the URL, so if any mod sees this and can fix it, I'd really appreciate, huh?

http://www.straightdope.com/classics/a3_199.html

you can humour me and copy-paste this once, right? (you should know that I'm feeling fairly stupid right now, as this has been explained to me repeatedly. I guess I'll never remember.)

[edit--I didn't need to enter code, at least not in my browser! hurrah!]

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It could be the lighting.

Under flourescent lighting, you can get the same find of effect without a camera, due to the pulse-like nature of it, as opposed to sunlight or incandescent lighting.

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One explaination I've heard is to do with your eye/brain frame rate. After certain speeds you start seeing the wheel going backwards because your eyes aren't updating fast enough.

It's the same reason why you aren't loking at your computer screen scrolling and updating. we can't process the information fast enough.

Let's say you have a four spoked wheel. To begin with the spokes are in the shape of a "+".
For an average car say a wheel is 0.6m diameter.
so it's circumference is: 0.6*pi = 1.9m
if the car is travelling at say 32m/s therefore the wheel is rotating at: 30/1.9 = 17rev/s

that value would have to be faster than the rate at which our brains can process the data for this phenomenon to occur.

Now, the spokes have obviously completed some part or more of one revolution of the wheel. If your brain updates the information from the eye at a speed at which the spokes have seemingly travelled more than 45deg (i'm guessing - probably further) but less than 90deg from their original position, then it will look like the spokes have travelled backwards.

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Airplane props look like there moving backward also. Why i dont know.

Um ...

Same Phenomenon, perhaps?

It's Not Limited to Wheels, ya' know?

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I have found (from napping in the passenger seat; don't try this while driving :P) that if your head is touching the window, the vibration of the car (I think that's what it is) can produce the "backwards turning effect".

I have just spent ten minutes watching various fans and things, and with a ceiling fan swiching between super-high-speed and high speed under incandescent light, I see a ghostly "image" of the blades turning slowly, in addition to the blurry circle presented by the blades. It's only for a few second, and then it goes away as the speed increases. I've seen this far more clearly with helicopter rotor blades, but whether it was filmed or not I cannot remember.

according to the article I cited, it's in part a function of the markings/spaces involved.

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Gillian

"Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"

"You can't erase icing."

"I can't believe it doesn't work! I found it on the internet, man!"

I can't speak to the brain scan rate. I would be surprised if the brain refreshes the whole image simultaneously at a regular rate, but what do I know.

But artificial lighting stobes with the power line frequency. I use this phenomenon when testing speeds of tape recording equipment. I have a spinning disc with lines on it, and they will strobe a pattern in the flourescent lighting. i am supposed to get out a neon strobe for this, but why bother... It is the exact same AC freq strobing either.

So if your spinning whatnot is indoors, chances are the lighting is strobing. Even putdoors, if there are artificial lights around, they will add their strobe effect to the otherwise steady solar lighting. Even on airport gateways there are high intensity lighting systrems running even in daylight often as not.

These effects can be seen in real life and are independent of any TV or film frame rates. Which have their own obvious effects.

Regardless of the source, the spacing of the spokes and their speed will have to interact with some form of strobe, whether it is film frames, flourescent lighting , or brain scanning. SO if you added more spokes, it would change the apparent speed of the phantom image, yes.

In movies, it has to do with the beta movement, the phi phenomenon, and perhaps persistence of vision (this one is disputed nowadays).
An early researcher was Joseph Plateau (with my apologies for the ironically bad visual layout of this site about visual media).

In real life, the reverse rotation effect, as it is called, is supposed to be rare, although my experience is that many people do experience it.

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Sometimes I can get the effect by making humming noises. Maybe it gets the eyes vibrating, thus producing the interference.

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IIRC, your eye/brain can interpret 28fps.

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Here there´s another example of this effect: The Technics desk.

It has a four dotted band along the edge of the plate. They all rotate at the same speed (from 33 to 45 rpm) but each band has different number and diameter of dots. 2 of them (I guess the bigger and more spaciated) go clockwise and the other 2 counterclockwise. It´s a fascinating view.

http://www.netmusic.pl/images/Kopia%20z%20deck.jpg

I would think that the electical frequency in a florescent light would indeed produce a strobing effect, but I'm sure I recall seeing the phenomenon outside in pure sunlight.

I am very skeptical about a human vision update frequency, and would love to read about such evidence.

I think a lot of blurring that is being seen is related to "persistence of vision." This is what is responsible for movies looking like smooth action rather than discreet frames. Any image that hits the retina will retain for a moment.

I've seen this effect as a shadow that follows behind a dark moving vehicle when the background is light. Or more obvious: just staring at a light & then turning the room to pitch dark will produce an image that hangs around for a long time.

I'd be very surprised if "persistence of vision" has been "busted". That combined with the idea that there is a human vision frame rate would be a significant scientific revision & I'd like to see a paper on such.

RBG

On the other hand, unless its possible to really analyze what I was seeing - maybe in every case there was some kind of florescent light involved which never made an impression upon me.

But I do recall pointing it out to a colleague of mine and both of us being surprised at what we were seeing - & both of us being film & video people.

No, this has to be real. Anybody else see this before? This has to be a well understood effect by lots of learned folks.

RBG

I spent half my live seeing the world through a video or film camera viewfinder, but I've never heard of the effect being seen directly by a human in sunlight.

Maybe in flickering streetlight, but not sunlight.

I read in an old astronomy book that a 18th Century scientist wondered if stars ever appeared to go completely "out" when they twinkled, and he figured out a way to see it. He observed them through a mirror and he saw them go out (dim to black) when he wiggled the mirror. Seems that with our "persistence of vision" we can't usually see this without a wiggling mirror.

I've seen the effect in broad daylight. Then again, I'm not the best test case as I have nistagmis. This my eyes twitch rapidly about 1 mm to the sides...all the time.

My gf, who doesn't have such birth defects, has also seen it in daylight.

That might be the reason you see wheels appearing to go backwards.

This is most noticeable in movies and on TV with wagon wheel that have spokes.

BA explained it fairly well.

A 16 mm movie camera runs at 24 frames per second and a TV camera photographs at 30 fps. A film camera has a shutter that is closed about half the time and open about half the time as the film moves inside the camera while the shutter is closed, stops while the shutter is open, then moves again while the shutter is closed. A TV camera has an “electronic” shutter that usually jumps from one video frame to another with no “closed shutter” time.

For film fps to be compatible with TV fps, some years back they invented a video device that photographs every 4th movie frame twice, and that converts films to 30 fps for TV. You can see that by still-framing a movie you record off TV, then advance the film one frame at a time.

If you have a wheel with four spokes, and the wheel makes 1/4 of a revolution while a movie camera shutter is closed, you will see the wheel as not turning at all, even though it is turning. Let’s say the shutter is open always when two spokes are vertical and two are horizontal. It will appear that way when you view the moving film and it will appear that you are seeing the same spokes always in the same horizontal and vertical positions. But actually you are seeing different spokes in the different positions. If the wheel doesn’t quite make a 1/4 turn during filming, then the spokes and the wheel will appear to be turning backwards.

Let’s say we take a four-spoke wheel and we paint the right horizontal spoke red, then we turn the wheel clockwise and photograph it with a film camera. If we turn it 1/4 turn in between each film frame, we will see the wheel appear to not turn at all, but we will notice that the red spoke seems to move around the wheel clockwise. The red spoke will first be seen as being at the horizontal right, then the vertical bottom, then the horizontal left, then the vertical top, then the horizontal right, etc. Of course, that reveals that the wheel is actually turning.

If we slow down the wheel a little, so that it doesn’t quite make a full 1/4 revolution turn while the shutter is closed, the wheel will appear to be turning backwards, however, we will notice that the red spoke goes from being horizontal right, to being almost vertical bottom, to being less than almost horizontal left, to being much less than horizontal top, to much much less than horizontal right. So that tells us the wheel is actually turning clockwise, but not quite 1/4 turn per movie frame.

Your eye twitching might also cause a similar effect when you see wheels turn in sunlight.