Why do the spokes of a wheel appear to reverse direction when they reach a certain velocity?

Answer 1

Jon's answer is quite accurate and complete, but his description could equally well describe how the same illusion can appear in 'real life.' In rooms where the lighting is from fluorescent bulbs, or on a highway with some types of intense lighting of the same type (more unusual), the illumination of the scene is not steady, but is flickering at a rate of 30 pulses per second (because of the frequency of the AC power, which is 60 Hertz, or cycles per second). Our eyes can't perceive this, just like we can't see the flickering in a movie displayed at 25 fps. However, if something is spinning, the flicker rate will behave just like any strobe light and create the same effect Jon described. Try this with a child's toy that spins, under fluorescent lights at home. Be sure to turn off any incandescent bulbs -- they don't flicker (because of the inductance of the filament ... but that's a different question).

Answer 2

What you are asking about is called 'aliasing'. The usual example is wagon wheels on old Wild West films where the wheel looks like it is going backwards.

It is to do with the frame rate of the displayed images in the film, or 'sampling frequency' in imaging terms. Basically, how many pictures per second. Aliasing is the result of undersampling, and here's why.

Imagine the wagon wheel being shown on a film at 25 frames per second (fps). When it's stationary, it looks exactly the same from one frame to the next. Now imagine the wheel turning slowly and you'll see the wheel turning. Now imagine one of the spokes is painted a different colour from the others and the wheel is turning slowly. At 25 fps you will see the painted spoke rotating slowly around, so you know that the wheel is moving forwards. Now return all the spokes to the same colour and spin the wheel a little faster. For the sake of simplicitity, let's say the wheel has 25 spokes. As wheel gets faster, spoke 1 manages to move on one spoke distance quicker and quicker. At 1 revolution per second, the wheel will appear to be stationary because spoke 1 is occupying spoke 2's position in frame 2, spoke 3's position in frame 3 etc. If the wheel goes just a bit slower than 1 revolution per second then spoke 1 will have got almost to position 2 by the time frame 2 is taken and the net result is that you think that spoke 2 has moved backwards because you can't tell spoke 1 from spoke 2.. The overall effect is that the wheel looks like it's going backwards. Hence 'aliasing'. If you increase the frame rate (ie. the sampling frequency) you'll see more frames per second allowing you to follow individual spokes more easily and your brain won't be fooled in to thinking that the wheel is going backwards. Eventually of course the wheel may approach a sufficient speed to cause aliasing the higher frame rate of course.

Answer 3

If you're talking about on TV or the movies, it's due to the framerate of the film and the rpm's of the wheel. THe film can't keep up with the velocity so that instead of succesive images of the wheel turning, the film will capture an image of the wheel after the fact and the result on screen makes it seem like it's going backwards.

If you're talking about the real world, I'm not sure if I've seen it happen with my own eyes.

Answer 4

on film?

It's the strobing affect of the frame rate and the how the images appear on the screen. In affect each subsequent image has a spoke in a position before the last.

Answer 5

...it's an optical illusion created in the brain as a result of visual information that can't be processed at such a high rate.... the eye is "seeing" the information, however, the brain is not processing it at the same rate...so...it backs up... all an illusion of sight and mind...

Answer 6

Apparently it's the brain being tricked into believing this, not sure why, but seen it being described on Discovery Channel once.

Answer 7

Bobby A has the correct answer

Answer 8

the strobe effect,