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Re: prove it objectively

In my opinion an objective demonstration of stroboscopic vision can be an
important first step toward learning about an interesting property of your
unusual eyes. You can do this, for example, at one of our AAPT meetings.
Bring a rapidly spinning wheel with removable spokes (large and shiny, if
you wish) and show that missing spokes can be counted under ordinary
illumination. Strips of white paper, or removable dots, glued to a wheel,
can be used as "spokes" in this kind of demonstration. ARE YOU WILLING TO

Ludwik Kowalski


What Ludwik shouts above is very close to my heart. I feel that the
connection between physics and reality has been culturally severed in
our western youth. Perhaps it has been television that was responsible;
I don't know. What I do know is that somehow we physics teachers are
also responsible because we are not recognizing the problem exists,
and we are the logical people to remedy it.

Place your hand, fingers open, between your face and the computer display
screen. Shake your hand gently. You are looking at a stroboscopic effect.
Do the same thing with one finger only. The effect is still there. Now
try the experiments in a room with only fluorescent illumination. The
effect seems to be there when I shake all the fingers, but it goes away
when I shake one finger. It is not as good an effect (if there is an
effect at all), and I think the reason is that the motion stopping effect
depends on there being a shutter speed or flash interval shorter than the
period of the illuminat - a smooth sinusoidal variation won't do.

Fluorescent lights give only a weak effect. The computer display works
because the phosphor persistence time is short compared to 1/60 s (or
1/75 s in the case of my display). An experiment I did last year is
illustrative of a related effect I think worth relating to the group.

A local high school student phoned me last year. he wanted to study the
motion of a "diabolo" top and asked me how he might measure its angular
velocity. I suggested using a video camcorder at high "shutter speed".
It worked like a charm. With the video and a freeze-frame advance VCR he
could measure angular displacements at 1/30 s intervals with ease. While
we were making the video I tried to get really high shutter speed, and
because we were doing this indoors I needed much more than available
light. I tipped a 300 watt tungsten halogen "torchiere" lamp (cheap at
the mall) over on its side to illuminate the top.

Well, the result was surprising! Because the frequency of the lamp
flicker was very slightly different from twice the field rate of my
camcorder (Color TV runs at a slightly different frequency from old black
and white) the apparent color temperature of the picture appeared to
drift slowly as the instantaneous lamp temperature varied. My 1/10,000
second shutter speed reveals clearly the appreciable variation in the
tungsten filament temperature. Even this incandescent source has a small
stroboscopic effect, something that was certainly a big surprise to me.
The effect would be greater for a tungsten halogen lamp than it would be
for a conventional nitrogen filled bulb because the peak temperature of
the former is greater, and it cools faster.

The student completed his IB project successfully and pleased me greatly
by enrolling as a physics major at Simon Fraser University, though he
was also accepted at other prestigious institutions in the US and the
UK, and he has the means to attend them. It is a thrill to me every time
I see the light turned on in these young people when they realize that I
am telling them and showing them things about their world!

That is what science teaching is all about.


(I didn't explain the difference between the multiple finger effect and
the single finger effect above. I leave that as an exercise.)