Why is it sometimes vehicle wheels appear to rotate in the opposite direction from travel on the motorway?

Question

Haven't you ever noticed this, how a decelerating high speed rotating part can appear to slow, pause and appear stationary and then appear to reverse direction, and then continue its deceleration?
Is it something to do with the frequency of light and the rpm of the object?
Why when you blink and stare at the rotating object can your eye take a stationary snapshot of the object?

All answers or theorys are appreciated, if only so that i stop standing at the roadside blinking at the traffic and scratching my head. Carl.

Answer 1

It's an effect called Aliasing. It's due to when the wheel reaches what's called the Nyquist frequency.

When the spokes are rotating at a rate just fast enough, one spoke will appear to be just behind the previous one, but overlapped enough to give you the illusion that the spoke has just moved backwards slightly.

This effect is valid for certain integral values of the Nyquist frequency, so at various specific speeds you will see this effect speed up or slow down. I tried to find you something on the web that illustrates this, but all the applets I found were static, and ridiculous. They would have confused you more than clarify.

Answer 2

What you see is an optical illusion. The wheel is turning so fast that your eye cannot keep the focus and compensates by literally missing a percentage of the wheels rotation. This gives the appearance of the wheel rotating in reverse.

To test this theory is quite simple. Close one eye and focus on ur index finger.. Now move the finger up and down as fast as you can whilst blinking with the open eye.. This gives the appearance of your finger either moving slower than it actually is or if you get the timing right the finger looks as though it is not moving at all.. its just the eyes inability to focus beyond a certain speed

Answer 3

This is called the stroboscopic effect, and happens when cyclic motion is observed at regular intervals. You could see it on film or television, because you are observing at fixed intervals (1/24 sec for film, 1/60 sec for television). If you are watching "live" you would not see this in daylight, but under artificial illumination, you would, expecially under fluorescent lamps. This is because these lamps actually switch on and off 120 times a second. The explanations given in other answers as to why the wheel seems to move backwards are correct. In particular, if the rotation brings the same point on a wheel to the same postition it occupied one illumination interval earlier, the wheel will appear stationary. Strobe lights have been used to analyze motion. A form of stroboscopic photography was developed by Muybridge in 1868. See http://en.wikipedia.org/wiki/Muybridge

Stroboscopic illumination was refined and used extensively by Edgerton. See http://en.wikipedia.org/wiki/Stroboscope

Answer 4

On film or with an electric light you will see the object blinking. This blinking is to fast for your eyes to see, but if the rotation is slightly slower than the blinking it will appear to move backwards.

Ps not all lights will do this only blinking lights, like a light bulb at home driven by AC current and I guess streat lights.

Answer 5

I'd be surprised if you could see this standing at the roadside.

It's an optical illusion caused in film making. Movie cameras for films actually take a lot of still photographs very quickly. There's still a gap between each frame. If the tire rotates 95% of the way around between each frame, it will look as if its rotating backwards at a slower rate when you view the film.

Answer 6

Your brain canonly cope with so much information at any one time.

Basically its like you see still images at 20 Hz. (20 cycles per seconds, or 20 snapshots a second)

The reason that some times it appears to rotate backwards is due to the timing of the individual snap shots you percieve.

Answer 7

That purely happens on television and movies. It does not happen in actual existence. The digital camera takes series stop action pictures. So at particular velocity, this is going to take pictures the place the spoke will look like it moved lower back instead of forward. As between the adult adult males above mentioned this is named stroboscopic effect. === think of of it this variety. instead of a spoke, shall we element of an extremely quickly turning clock. the 2d hand strikes forward. you spot it at 12 o'clock. A chop up 2d later yet another photograph is taken and the 2d hand is now at a million o'clock. So your strategies places that at the same time and you spot the 2d hand pass foward. yet think of that the 2d is shifting plenty quicker and instead of taking the 2d picutre at a million o'clock place this is definitely moved to 11 o'clock place. So the 1st photograph could be at 12 o'clocl positin , the 2d photograph could be at 11 o'clock place. The strategies places that at the same time and thinks the 2d hand has moved lower back. this is it. sturdy success...

Answer 8

Your eye to brain speed is only finite and sees fast moving objects as a blur when they start to slow down your eye starts to see it more clearly but your brain can't ajust quick enough to make perfect sense of the images. Only when it reaches a critical speed (slower) can your brain make total sense of the images and you see it properly.

Answer 9

It looks that way watching on tv because of the speed of the film, but I haven't noticed it in real time on the street corner.

Answer 10

It's an optical illusion caused at certain speeds of rotation.