*************************************************
Ken Fox writes:
Looking at a ripple tank and introducing using a stobe to determine
frequency, the discussion went to seeing wagon wheels in movies seeming to
roll backwards. Students maintain that they have seen this in cars with
naked eye. I don't inderstand the mechanism of how that would happen, yet I
in retrospect, I think I have seen it, too. Any ideas?
*************************************************
I believe this effect is due to vibration. It is easily seen when looking at truck wheels in the
mirror of the car while driving. I once saw it from an aeroplane looking down on a
helicopter - the blades strobed. I read that you can see the refresh line in the television by
humming at the appropriate frequency, but I have never tried it.
The vibration must cause the eyes to refocus regularly. The focused spot is refreshed
regularly and it strobes. The constant focusing also requires a 'refresh' but that is ignored
by the brain.
Michael McInerney
Rose-Hulman Institute of Technology
Terre Haute, IN