Blade illusion?

Question

When propellors move fast, we are able to (kind of) see each individual blade move slowly, usually bluish on colour. At times, it is in the opposite direction of the actual blade movement.

Please answer with reference to the human eye and it capturing images 1/16th of a second rather than video's 24 fps.

Answer 1

This only ever occurs when something is capturing images non-continuously.

For example, a video camera might capture 30 images per second, so, looking at a video of a propeller, you might see the propeller moving exactly 1 revolution each time the video camera captures a frame. It would appear to be motionless.

The human eye captures images continuously, but if you use a rapidly-flickering light (such as a fluorescent), you can see this effect.

If you have steady light (sunlight or incandescent bulbs) and are viewing it with your naked eye, you will never see the effect.

Answer 2

It's a stroboscopic effect. When a rotating electric fan is illuminated by a flashing light source (called a stroboscope) so that a flash arrives whenever a fan blade passes a fixed position, the blades will seem to stand still. This is a useful way of observing fast-moving objects such as machinery or insect wings. If the flashes occur less frequently, the object will seem to move slowly in its actual direction or backwards. Since electric lights are 60 cycle current this too is a sort of strobe light and the effect is similar. The eye responds to 1/10 second changes. So anything faster will seem blurred or in motion. this is the basis of animation and motion pictures in general.

Answer 3

lithiumdeuteride's answer is correct, but I thought the brain did process images captures from the eye at a given rate. I can't think of any references for that, and will have a look!

lithiumdeuteride, do you have any references as to if the eye and brain do really display continually changing images?

Cheers,
Ben

Answer 4

IT IS CALLED RETROGRADE MOTIOn... it happens even with the observation of planets.