Why do car wheels appear to spin backwards?

Why do car wheels, helicopter rotor blades and airplane propellers appear to spin in reverse at high speed?

Why do car wheels seem to spin the other way?

A YouTube video showing a camera being accidentally dropped from a plane full of skydivers and going into a spin got us thinking: why does the image begin to appear relatively stable when the camera is spinning at its fastest?

GoPro dropped from plane

A camera dropped from a plane goes into a spin before landing next to a curious pig

This phenomenon is linked to the same effect that we've all seen while staring at car wheels on long drives: they seem to start paradoxically spinning in the opposite direction when at high speed.

This optical illusion is commonly called the wagon-wheel effect, so-called for being recorded in old Spaghetti Western films, and relates to the stroboscopic effect of video frames. You may have also noticed it with helicopter blades and aeroplane propellers.

The Effect Explained

Old video cameras record moving pictures as a series of frames on a film. Similarly, modern video cameras record a series of digital images, usually at around 25 or 50 frames per second. The film is then fed through a projector or the series of digital images are flicked up on a screen at such a rate that your eyes perceive the series of frames as one continuous moving image.

Aeroplane propellors seem to spin in reverse at certain speeds

Aeroplane propellors seem to spin in reverse at certain speeds

As such, if a blade on a clockwise-rotating propeller was recorded on a video camera at 12 o'clock in the first frame, then 9 o'clock in the second frame and so-on, rotating at three quarters of a revolution each frame, when played back the propeller will appear to be rotating backwards - counter-clockwise at one quarter revolution per frame.

You could also make the propellor appear stationary if the blade rotated the full 360 degrees with each frame, back to the 12 o'clock position.

But does the human eye perceive motion in the same way that cameras record frames?

Motion Perception

Our brain perceives motion in two ways; first-order or beta movement perception and second-order perception.

First-order perception is much like the way video cameras record motion, with the brain working out if an object has moved by the change in stimulus over time on neighbouring light-receptive cells on the retina. Second-order is more complex and is down to the contrast and texture of an object.

Although the wagon wheel effect is easier to see when recorded on video, due to the restrictions of frames, first-order motion perception explains why we can sometimes see it with our own eyes.

William Rushton found that he could replicate the wagon wheel effect by vibrating his eye! If he hummed at the right tone, the vibration of the eye will effect motion perception in a similar way to the frame rate of a camera.

Another way to do this is to apply a strobe light to the spinning object, artificially creating the "frames" much like a video camera!

The Video

So, back to the camera falling out of the plane. Why does the image appear to stabilise as the spin gets faster?

This is because the spin of the camera as it plummets to Earth resembles that of the frame rate recorded by a camera or the first-order motion perception that we perceive.

If the camera was recording video at 25 frames per second, and the camera spun faster and faster until reaching approximately 25 revolutions per second (or a multiple of 25), each image frame recorded would be captured at the same angle, giving a relatively stable image - although not perfect, as you can see.