Date: Thu, 27 Jul 2000 16:32:08 -0600

Author: Jerry DiMarco

Subject: Re: light below surface, PIRA # 6A44.50



According to the write-up in Freier & Anderson, which is the reference
for that demo, the powder is used to show where the beam is coming out of
the water. The way the picture is drawn, the jar appears to be very large
- 8" high and about the same in diameter. The bulb is placed at a level
such that the beam coming out of the water (the central part of the beam
that is less than the critical angle) is about 3"-4" in diam. There is no
mention about light effects on the bottom.
Regarding your setup:
1) Are you using a miniature lamp with a small filament?
2) Is some part of the apparatus causing reflections, or is there too much
light in the room?
3) Could the shadow on the bottom be from the lamp base?
4) Why did you choose lycopodium powder? The write-up just says "a
powder". Do you have a different set of instructions?


At 03:23 PM 7/27/00 -0400, you wrote:
>Will you let us in on the big secret?
>A student and I are trying to do the "Light Below Surface" demo, PIRA no.
>6A44.50. This is on the PIRA 200 list. This is the demo where a
>light bulb is placed underwater and lycopodium powder is sprinkled onto
>the surface to illuminate the disk of light on the surface
>where the light breaks through the water.
>(1) Is is right? That is, is the lycopodium powder supposed to show the
>disk of light right on (or rather, immediately above) the water?
>(2) I've checked perhaps a dozen demo web sites, and nobody seems to do
>this demo. Why is it a PIRA 200 demo?
>(3) We haven't been able to get his demo to work satisfactorily. When we
>put a bulb underwater, using a squat, round beaker (so that light exits
>the sides with minimal reflection) and positioning the base so that no
>light shines directly downward from the bulb, we're able to see a dark
>disk on the bottom, which represents where the light has not reflected
>from the surface onto the bottom. The edges of this dark disk are fairly
>distinct. There should therefore be a bright, lighted disk on the
>surface, with a diameter about half that of the dark disk on the bottom.
>However, the lycopodium powder fails to illuminate this disk clearly.
>We've tried a range of thicknesses in coating the surface, without much
>luck...the disk's edge is not distinct. Any suggestions? Anybody
>make midget ping-pong balls?
>(4) It seems to me that the blackboard optics demos for
>total internal reflection, or a laser shining through a large light
>pipe, are just as good for illustrating the point of the light
>under water demo. Am I missing something?
>Thanks to all,
>Steve Wonnell


Jerry DiMarco, Instructional Lab Supr. ph: 406-994-6161
Physics Dept., Montana State Univ. fax: 406-994-4452
Bozeman, MT 59717-3840 e-mail:

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