# Spinning wheels and diamond wire fences

1. Jul 18, 2007

### LHarriger

So I have two (somewhat) related questions.

1) Why is it wheel rims (or spokes) sometimes appear to spin backward? Everytime I see this it drives me crazy. I just cannot figure out how a wheel spinning real fast in one direction appears to be spinning real slow in the other direction.

2) So the first question I expect everyone has experienced. This second phenomenon might have escaped notice by most but it is easily reproducible. So if you havn't noticed it before, check it out. Next time you are driving in a car parallel to a diamond wire fence (like you see around playgrounds, schools, junkyards, etc.) take a look at it. Your line of sight should be roughly perpendicular to the plane of the fence (this is not really necessary, but it is the best angle I've found for causing the effect). Don't wreck. The fence should be a quick moving blur and then BAM! you will notice that the diamond shape geometry of the wires gets blown up and superimposed on the passing blur of the actual fence. Kind of like one of those 3-D out of focus pictures. Try it, I swear you will see it to. What is this effect?

3) While we are at it, how do 3-D pictures work also?

2. Jul 18, 2007

### Staff: Mentor

-1- You only see this if you are looking at a recording of the wheel spinning. You are seeing it because the video recording has a frame rate something like 30 frames per second, so you are seeing a sampled version of the actual physicsl motion. If the rotational speed happens to get close to the sampling speed, the apparent wheel rotation will slow down and stop, then start going backwards. It's similar to when you use a strobe light to observe some object spinning or moving. You can also see it by waving your fingers in front of your TV set -- you'll see discrete shadows of your fingers as the TV refreshes the screen.

-2- Weird. I'll have to try that (as a passenger in the car, not the driver).

-3- You need two images for your two eyes in order to get depth perception. So the image has to be photographed from slightly different angles (to emulate the eye separation), and then the two images have to be presented to you at the same time, with the appropriate image going into each eye. You can do this with color separation (red/green glasses), or polarization separation, or some other techniques. Or you can use holography, which would be a whole other thread (in fact there are lots of holography threads here on the PF).

3. Jul 18, 2007

### LHarriger

This makes good sense. It is the same reason (CRT) computer screens bobble up and down when viewed on a TV (such as while watching the news). However, I could have sworn that I've seen this in real life. Maybe my mind is playing tricks on me....

How does color or polarization seperation isolate each image to a specific eye?

4. Jul 18, 2007

### Staff: Mentor

Another possibility is that you were seeing the "spinner" wheels. Those suckers will confuse you! I thought I was halucinating the first time I saw them on a car sitting at an intersection (still spinning)!

Typically you wear glasses that switch the polarization of both lenses at the display frame rate, and the display switches back and forth between the left and right images every other frame. So one eye is blocked while the other sees its image, and then it switches.

You can also have projection systems that put up the two images on a screen with two polarizations. That's how it's done at the Disneyland 3-D theaters, I believe. The glasses they loan to you have two different polarizations (I don't know if they are linear or circular, though).

5. Jul 18, 2007

### Danger

There's a neat alternate way to get the 3-d effect, sort of the reverse of those 'magic mural' posters. I saw it in Starlog (or maybe Future Life, the sister publication) a few decades back.
You still use parallax-shifted pictures from 2 cameras (or a dual-lens camera), but put them on the wrong sides. Cross your eyes to merge them into one 3-d image between the two 2-d ones. You can actually move your eyes to scan around the image once you're 'locked in'.
It also works great for those 'spot the differences' between 2 pictures games. Anything that's the same looks normal, while the differences appear as semi-transparent and stand out immediately.
By the bye, LHarriger, the type of fence that you refer to is known as 'chain-link'. I've never noticed that effect, but I've always kept my eyes on the road. Now that I have W and she does most of the driving, I'll try to get her to go past one to check it out.

Last edited: Jul 18, 2007
6. Jul 19, 2007

### belliott4488

I think I've seen that effect that you mention with the fence, but if so, then it requires two fences - one closer and one farther away, which you see through the first one. You can also see the same effect looking through two screen windows or doors.

I believe that what you're seeing is an interference effect. Although the spacing of the wires in the two fences or screens might be too small for you to see, the places where they line up "on top" of each other are much farther spaced, so you can see them. What's happening is that the farther fence/screen has an apparent spacing that that is smaller than the one that's closer. If you're familiar with the phenomenon of "beats" between two waves of different wavelengths, it's the same thing. The two patterns line up to reinforce each other every X wavelengths, or repetitions of the fence pattern.

7. Jul 19, 2007

### Danger

That sounds reasonable, Belliot. I've noticed that effect with screens; it's almost a moire pattern. The only place that I can think of finding two chain-link fences that close together is at a corner, where you can see one 'leg' through the other. If the speed of the observer isn't critical, I'll check it out when I walk through the schoolyard on my way to work today.

8. Jul 19, 2007

### ice109

why do you think you can only see this in a recording of the wheel? i've seen this many times IRL, i think..., and seeing as how your eyes only refresh at about 60Hz so anything rotating close to that frequency will exhibit the same effect

9. Jul 19, 2007

### Danger

More like around 10-14, and there's a big difference between how fast the optic system works and how fast the various areas of the brain process the input from it. Television refreshes at 30 Hz because that's fast enough to appear as a 'seamless' moving image. If you have a light flashing at anything over 16 Hz, it will appear to be constantly on. If your eyes refreshed at 60 Hz, you'd be able to see your house lights flickering as the AC current reverses.

10. Jul 19, 2007

### ice109

i can see my my monitor refresh at 60Hz, and light don't completely turn on and off when the current reverses.if they turned on an off i bet you would be able to tell

11. Jul 19, 2007

### Danger

I can't explain what's going on with your monitor, unless maybe it's scanning inaccurately. AC lights do turn completely on and off. The filaments, however, don't cool down much during the switchover so they just dim rather than go completely dark. Think about it: if the current didn't completely shut off when it reverses, transformers wouldn't work.

12. Jul 19, 2007

### mgb_phys

You can see the monitor flash at 60hz and sometimes see the wheels frozen.
What happens is that your eyes/brain don't run at an accurate clock rate so sometimes it happens that your eye refresh rate and the monitor get in sync for a while so that on subsequent eye 'exposures' you happen to both be on monitor blank times.
A similair clock jitter design is used on high speed oscilloscopes to capture details much faster than the official samplig rate.

You can often see this better out of the corner of your eye which is designed to look for fast changes.

13. Jul 19, 2007

### Staff: Mentor

Cut it out, folks. The human optic channel is not a sampled system. It's a continuous analog system. There is no "sample rate" for the human eye or brain. Jeeze.

14. Jul 19, 2007

### mgb_phys

The eye is neither/both a sampled and continous system. Your eye moves to scan the fovea over different parts of the scene and your brain tries to prioritise the most important parts of the scene - there is a lot of complex software to produce what you see.

As an experiemnt.
Move a flashlight or a laser pointer quickly - you will see a series of discrete points of light not a continous stripe. ( do this without AC lights so there is no question of strobe effects)

15. Jul 19, 2007

### Danger

There's no such thing as a 'continuous analog' signal where neurons are involved. A distinct 'refresh' rate exists simply because the potassium/sodium imbalance must be recharged between firings. That, however, is not the main point. It's the conscious brain that can't detect a 16+ flash rate.
A lot of pilots have been killed because of that effect, which is why I researched it in the first place over 30 years ago. The brain's alpha rhythm is 7.8 Hz, or approximately 1/2 of the highest detectable flash rate. A light at that frequency appears to be steadily on, but the visual cortex still fires in response to every flash. It can stimulate an artificial alpha that essentially induces an epileptic-like siezure. For some unknown reason, a red-orange light source has more impact than other colours. All of the foregoing has presented itself to the aviation community in the form of 'flicker vertigo'. An unwarranted number of pilots were crashing during sun-set landings, and that turned out to be the reason. At a typical power setting for landing, the prop is cutting the (red) sun approximately 16 times per second.

16. Jul 19, 2007

### LHarriger

Easy with the big vocabulary, maybe tomorrow you can help me learn my farm animals. (Kidding of course.):tongue2:

When I first noticed this effect it was with two fences. However, since then I have observed it for only one fence. You have to be moving pretty fast though (or atleast I think). In any case I have only noticed it on interstate travel and similiar speeds.

If you move your eyes quickly from the top of your monitor to the bottom then I have sometimes seen a moving line flashing downward. I expect it is same effect as watching a fan blade rotate by moving you eyes in a circle.

17. Jul 19, 2007

### DaveC426913

It can happen in any circumstance where the light pulses, for example under a fluorescent light.

Eyes do not have a refresh rate.

While they do have a retention rate, it will not work the way you describe. If it did, you could look at a moving piece of film without benefit of a projector.

Monitors make 2 passes in 1/60th of a second, so you're seeing 1/30th. And seeing a flicker is not quite the same as retaining the image.

Critical to understanding this illusion is the fact that your eyes do not move continuously. They jump rapidly in a series of jerky movements. This acts somewhat like the shutter of a projector.

Last edited: Jul 19, 2007
18. Jul 19, 2007

### Staff: Mentor

Thanks, Dave. I was about to move this thread from General Physics to Skepticism and Debunking

19. Jul 20, 2007

### Danger

Excuse you?! :grumpy:

Flicker vertigo is a very real, deadly phenomenon. A lot of damned good pilots died because it wasn't recognized. A modern update of it is the problem that some kids have with video games causing a similar effect, which has just made the news in the last few months.