Wagon Wheel

[ezapf]

Why do spoked wheels sometimes appear to spin the wrong way?

[bernardo]

Presumably you are referring to the way they appear on TV and in films.

 

It is because of what is called an interference pattern between the spokes on the wheel and the frames of the film.

 

Imagine that there are 24 spokes in a wheel, and 24 frames in one second of film. 

 

If the wheel is moving round at one rotation per second, then each spoke in the wheel will move forward to exactly the position where the spoke in front of it was in the previous frame (i.e. one twenty-fourth of a second ago).  The film will make it look as though the spokes are standing still (because the spokes moving round the wheel match exactly with the frames of film going through the camera) (in other words, the film will make it look as though spoke 1 is where spoke 1 was previously, whereas in reality spoke 1 will be where spoke 2 was previously).  

 

If the wheel is going slightly faster than one rotation per second, then each spoke will move forward to just beyond where the previous spoke was in the previous frame.  The film will make it look as though the spokes are moving forwards slowly (in other words, the film will make it look as though spoke 1 is just beyond where spoke 1 was previously, whereas in reality spoke 1 will be just beyond where spoke 2 was previously).

 

If the wheel is going slightly slower than one rotation per second, then each spoke will move forward to just before where the previous spoke was in the previous frame.  The film will make it look as though the spokes are moving backwards slowly (in other words, the film will make it look as though spoke 1 is just behind where spoke 1 was previously, whereas in reality spoke 1 will be just behind where spoke 2 was previously).

[bernardo]

Look out for a film of a car or coach slowing down - it will look as though the spokes are going forwards; as the car slows down the spokes will appear to slow down and stop, and then go backwards.  Then it will become a blur, and when the car becomes slow enough you will be able to see the spokes moving slowly as they really are.



How it appears on film all depends on how far the spokes move in between each frame.  Sometimes you can see a wheel on a car which has got two separate sets of holes at different distances from the middle, in which one set of holes appear to go forwards and the other set appears to go backwards on the same wheel (!).  This is because there are different numbers of holes in the circle, so they interfere with the frames of the film in adifferent way.



The same sort of interference patterns happen quite often in various types of physics - sound and light and colours and waves as well as film.  For example, the colours you see on a film of oil or in a rainbow depend on the way the light interferes with itself after being reflected off the two surfaces of the liquid!  If you do a google search on "interference patterns" you should be able to find loads of stuff all about it.

[bernardo]

Another interference pattern you can make easily is if you hold a pen (or a finger) in front of a TV screen or a computer screen.  Wave it from side to side quickly in front of the screen and see what happens!  Then do it more slowly and see what happens.

[smudge]

Hi bernardo, I have been trying the waving of the pen & finger thing, then a whole hand in front of the computer screen!  I bet everyone will be trying it now!

 

My 4 year old Grandadughter is fascinated & can't wait to show the teacher in I.T.!

[ICEMANSAV]

Amazing Bernardo! Just looked silly waving a pen at work though!

[MORELLO]

So I'm waving this pen looking a tw@t - what am I supposed to be seeing?

[gazzawazza]

Something (along the same lines) which I discovered for myself when I was an ickle kid...  Stand at the side of the road when a car is coming and follow the car with your eyes.   Just as it gets about level with you the wheels appear to stop but only if you continue to 'pan' with the vehicle. 

[bernardo]

MORELLO: you should be seeing a series of "flashed" images of your finger/pen which are caused by the screen flashing loads of times per second, and leaving a shadow where your finger blocks it out.  The fact that you see a series of distinct images of your finger (instead of a blur) shows that the computer screen or TV screen is flashing individual frames rather than showing a smooth continuous beam of light.

[ezapf]

This works in front of a CRT TV or computer monitor, but not with an LCD flat screen.