name of the optical effect where a wheel appears to be turning the opposite way?
488848
Temporal aliasing\n\nTemporal aliasing is the term applied to a visual phenomenon also known as the "stroboscopic effect". It also accounts for the "wagon-wheel effect", so called because in video or motion pictures, spoked wheels on horse-drawn wagons sometimes appear to be turning backwards.\n\nTemporal aliasing is one example of a range of phenomena called aliasing that occur when continuous motion is represented by a series of short or instantaneous samples. It occurs when (a) the view of a moving object is represented by a series of short samples as distinct from a continuous view, and (b) the moving object is in rotational or other cyclic motion at a rate close to the sampling rate.\n\nExplanation\nConsider the stroboscope as used in mechanical analysis. This is a "strobe light" that is fired at an adjustable rate. Suppose you are looking at something rotating at 60 revolutions per second: if you view it with a series of short flashes at 60 times per second, each flash illuminates the object at the same position in its rotational cycle, so it appears that the object is stationary. Furthermore, at a frequency of 60 flashes per second, persistence of vision smooths out the sequence of flashes so that the perceived image is continuous.\n\nIf you view the same rotating object at 59 flashes per second, each flash will illuminate it at a slightly different part of its rotational cycle. Fifty-nine flashes will occur before you see the object in the same position again, and you will perceive the series of images as if it is rotating once per second.\n\nThe same effect occurs if you view the object at 61 flashes per second, except that each flash illuminates it a little earlier in its rotational cycle and so, it seems to be rotating backwards.\n\nIn the case of motion pictures, action is captured as a rapid series of still images and the same stroboscopic effect can occur.\n\nWagon-wheel effect\nSee main article: Wagon-wheel effect \nMotion-picture cameras conventionally film at 24 frames per second; however, the wheels of a vehicle are not likely to be turning at 24 revolutions per second, which would be extremely fast. However, suppose each wheel has twelve spokes and rotates at only two revolutions per second. Filmed at 24 frames per second, the spokes in each frame will appear in exactly the same position. Hence, the wheel will be perceived to be stationary. In fact, each photographically captured spoke in any one position will be a different actual spoke in each successive frame, but since the spokes are close to identical in shape and color, no difference will be perceived.\n\nIf the wheel rotates a little more slowly than two revolutions per second, the position of the spokes is seen to fall a little further behind in each successive frame and therefore the wheel will seem to be turning backwards.\n\nDerivation\nThe reason it is called "aliasing" is that in electrical engineering, when a continuous signal is replaced by a series of samples — say, a 24.1 Hz signal is sampled 24 times per second — the result seems the same as if a 0.1 Hz signal were sampled 24 times per second, so 0.1 Hz is said to be an "alias" of 24.1 Hz.
11142
Spin the other direction at the same speed for the same amount of time.
690292
The direction of rotation is the direction you look in when you see clockwise rotation. It's the same as the positive axis of rotation, sometimes called the spin axis. (It's also the direction a right-handed screw moves in when turned clockwise.) It may initially be hard to imagine rotation as having a direction like this, but consider the inconsistency that arises when you think of it as being in the tangential (rolling) direction as you do now. What distinguishes the motion of the bottom of the wheel from the top, front or back? These all point in different directions, and there is really no reason that one should be any more of an indicator than another. It's as if rolling it on the ground is somehow a higher "priority" than on the ceiling. These directions, and the direction the wheel rolls in, all actually indicate translational (nonrotary) motions that result from rotation, not rotation itself.
32274
it's an optical illusion
162299
It´s because your eye doesn´t receive light in a continuos way. Our eyes are "taking pictures" several times a second, and we have the perception of movement. When a wheel turns fast, suddenly the speed sinchronizes with your eye so first it looks as it is stopping, then it goes backwards, and then again seems to turn forwards.\nThis happens when the speed is changing.
170333
What happens here is the reverse of dispersion that is the spectrum of colours obtaines which u see as the colours of rainbow or light emerging out of prism. When u spun the top all the colours get mixed to form their parent colour which is white. The same phenomenon is evident when wheel decorated with all the colours of rainbow is rotated fast.
18257
you were correct.  It appears that the howstuffworks diagram showing the bicycle mentioned in another answer is in error in that the bicycle would be upside down as shown but the tires and handlebars should not be reversed the way shown.  For example if you are looking at a nearby shrub and a distant tree, both would be upsidedown on your retina.  But the order of objects would be tree, shrub, focal point of eye, shrub image then tree image.\n\nAn experiment was conducted where people wore special glasses that inverted objects as the eye does.  After a while they learned to see correctly again and when they took the glasses off everything was inverted until they re-learned to see correctly once again.
317749
Yeah, it well happens in real life, not just on TV.\nIt is due to the persistence of vision, and it is temporary because it depends on the relative speed of the wheel or the fan or the tyre, whatever you are talking about.
1091417
Fascinating question! The surface of the vibrating object would alternately cause reflected light to red shift and blue shift, but that's about as far as my thinking can go. Would the shifts cancel each other, rendering the object invisible?
634672
This one has a very interesting answer.\nI am going to try drawing some sketches here. Please bear with me...\n\nAssume that | represents a spoke that is rotating and is currently vertical.\nNext assume that after some time, it goes to position /\nExtending this logic, you see something like this in case of a complete rotation\n| -> / -> -- -> &#92; -> |\n\nNow assume that you take pictures of the whole thing at some interval such that this interval is 3 times slower than what I showed you.\nSo what you would see would be\n| -> &#92; -> -- -> / -> |\nBasically you need to follow 3 arrows and keep cycling around to get to the image.\n\nif you notice, my second case seems to be rotating in the opposite direction of what I had drawn earlier.\n\nThe wheel rotates at some speed. The film is essentially a sequence of still pictures cycled through at some other speed. This difference of speed creates this impression of backwards motion!\n\nHope, I explained it well enough for you!
704494
Have people never seen a real helicopter close by.... of course the rotors appear to go backwards... It is an optical illusion but it is exactly the same as a spinning bike wheel. When it reaches a certain speed the eye See's a part of it slow down and then it appear to go slowly backwards...... until it's just a blur again....\n\nI will go look for what the term for this is called and will be back with an edit shortly ;o)\n\nOK Back. I found this....\nImagine a clock with just a minute hand. If we were to look at the clock every 59 minutes it will appear to have gone backwards by one minute each time and if we look every sixty minutes it will appear not to have moved at all! So to accurately observe the minute hand in motion we must look at the clock more frequently than every sixty minutes, in fact we must look at it every thirty minutes. This is known as Nyquist's theorem.\n\nGraham
1103211
It doesnt reverse. You see yourself how others see you. It's not like the light waves are switching sides. . . it's the fact that they're staying on their own side that makes it look like they switched!!
1163276
The angle the vehicle was traveling
1165249
This is the part of the wikipedia that I understood.  I can explain it to you if you want to send me a message.  I think it makes alot of sense.\nNow let’s see what happens to our accelerating particles when a torque is applied to the spinning wheel. Assume the axis of rotation created by the torque is through the center of the wheel at 90 degrees to the primary rotation of the wheel. Let’s look at a particle that is on this axis of rotation. Since the particle is on the axis of rotation there is no direct motion applied to the particle at the instant of the applied torque. But let’s look at what will need to happen at the next moment in time. The particle is now going to be forced to curve again. This time in the direction of the curve so as to accommodate the tilt of the wheel. Now we have a particle that is already moving and it wants to keep moving in a straight line. So the particle will exert a force on the wheel. If you look at a particle on the other side of the wheel you will see that the force of the second particle is in the opposite direction of the first particle. That pair of unmatched forced is what causes the precession torque that is 90 degrees to the applied torque
844842
I'm pretty sure it's called the Coriolis Effect. You can look it up.
38582
I realize I am out of my league as far as physics,, but the image reflects directly back to the source in front of it. For the reflection to appear upside down or vertically mirrored the light would have to be bent.\n\nok looked it up.  It is because of the predicatable behavior of light known as the law of reflection
848599
No. The velocity of the wheel is changing as the car moves (because velocity indicates direction). So as the car moves forward, the velocity of the surface of the wheel is continuously changing.