#2             80. TWO EXTRA VERTICES             

We had been watching a film about early Adelaide.

It was certainly interesting to compare the pace of life in those early days with the hectic rush of today.

I was particularly curious about the early model cars which had the rims of the wheels joined to the axles by large spokes.

For, not only did these cars travel forwards rather slowly by today’s standards, but often their wheels appeared to be revolving backwards!

Now I assume that this is an optical illusion.

But why does it occur?
HINT 1

Think about how the film is made. The motion occurs as a result of a sequence of ‘frames’.
HINT 2

Think about how the wheel might look in two successive frames. What would make us believe the wheel was turning forwards? backwards?
SOLUTION

Imagine the wheel is rotating in a clockwise direction. If in the diagram the solid spokes appear in frame 1 of the film, and the dotted spokes give the position in frame 2, the eye naturally interprets this as anti-clockwise motion.
EXTENSIONS

1. Investigate the workings of the old time projector. You will find that the Reuleau triangle was put to good use.

2. Can you think of other situations where the time lapse in a film might cause an unexpected effect? For example, think about how a TV picture appears when shown as an object, as seen on TV.