[Seany (OP)]

I'm sure you have seen this!!

Why does it seem to spin backwards?

[DoctorBeaver]

It's a kind of strobe effect. You get the same with aeroplane propellors.

If I remember correctly, it's caused either by our eyes not capturing images that quickly or our brain not processing it fast enough. Either way, what happens is that we don't see the motion as continuous, but as very rapid snapshots. If the period between each snapshot is slightly less than the time it takes for the wheel to completely turn once, then each position on the wheel will appear to have retarded slightly. Add these all together (similar to an old movie) and what we see appears to be the wheel spinning backwards.

[Seany (OP)]

Yes.... But it's not de-cellerating (or whats the opposite of accelerating).... Even if it's going at a constant speed, say 100mph, it still moves!

[DoctorBeaver]

It has nothing to do with acceleration or deceleration. Take the example of a 3-spoked wheel. If 1 of the spokes is at the bottom when our first "snapshot" registers but a few degrees back from the bottom (in other words, it has almost, but not quite, completed 1 revolution) when the 2nd snapshot registers, then for the 3rd snapshot it will appear a few more degrees backwards, further back for the 4th, and so on. All these snapshots added together give the effect of the wheel revolving backwards.
[diagram=257_0]

In that (dreadful) diagram, the wheel is revolving clockwise but the yellow line appears to be rotating in the opposite direction to the wheel as it is slightly further retarded for each snapshot.

[Karen W.]

That is cool to know.. as I often wondered that my self.. It always puts me in mind of those silly Illusions that you look at.. with the circles, after looking for a moment or two the circles start moving or spinning backwards and forwards sometimes! It is weird!

[dentstudent]

This effect is somewhat enhanced when you're out at night. The lights from the streetlamps flicker at 50Hz (I think, but it doesn't matter - the point is that it isn't a constant light) and when the wheels are turning at the same rate (or a frequency harmonic to 50Hz), they appear to be stationary. If the car even immperceptably speeds up or slows down, the relation of spin of the wheel to the rate of flicker of the lights will change, giving the effect of going backwards or forwards.

[daveshorts]

It is a much stronger effect on film or TV because the camera is definitly aquring snapshots at 25 or 50 times a second.

[lyner]

The effect has a posh name; it's called temporal sub - sampling.  It produces 'aliases' in time , just as aliases in space  (wavy fringes) are caused by pixellation  (sub sampling) of regular  fine structured patterns on digital pictures and scans.
I've never come across it in ordinary daylight;  The eye doesn't 'sample' a scene in time like a tv or film camera does - i.e. at a regular rate, so the classic 'wagon wheels going backwards' effect  (as well described by Dr B) is not likely to be seen when the light level is constant.
There is a lot of 'lag' in our vision - which is why cinema  and tv work.    Most  movements that are too fast just appear as a blur. But the brain is a funny thing. . .

[DoctorBeaver]

I was unfortunate enough to be on an old WWII Dakota flown by Air Burundi & I noticed the effect when I looked at the prop out of the window (I could see it through the gaps in the fuselage too - but that's another story).

[lyner]

I might suggest that the effect on the appearance of the propellor was that the vibration from the engines was affecting your vision and 'modulating' what you were seeing.
I have had a related experience when walking towards a computer monitor (CRT) at a distance of, perhaps 10metres. The grey screen appeared to be flickering at each footfall - I could repeat this and other people saw it too - and with a number of different monitors. Clearly a small mechanical shock on the brain was doing something to my vision.

[DoctorBeaver]

hmmm, I'm not sure that was the reason. Surely, that would just cause a judder rather than temporal sub - sampling.

[lyner]

Yes. But the resulting modulation of the (percieved)  image,  caused by this engine  judder, could give the same effect - just like the  mild brightness modulation with 50Hz  filament bulbs gives you strobing effects. Sampling can be regarded as modulation, too and produces aliases or intermodulation products.

[DoctorBeaver]

Yeah, I suppose it could have been that.

[Soul Surfer]

You tend not to be aware of the fantastic amount if image processing and stabilisation that the human eye and brain does.  Where you are moving and something else is moving like the image on a CRT monitor this stabilsation breaks down and you become aware of the real image instability on your retina  that is normally taken out by the image stabilisation processes in your brain.