Date: Fri, 15 Mar 2002 14:45:55 -0800

Author: Doug Johnson



Hi Eric and others,
Yes, we are talking about the bright spot in the center of the shadow.
I guess it is just the fine tuning so that everyone can see it, is my goal.
I found that with my laser, the smaller the object, the brighter the spot.
I am always looking for a better way to show the same thing. Thanks for
all the details. I plan to try it out soon.

Thanks again to everyone for your comments! ....Doug J.

>It's about 4.5mW HeNe, which I expand with a 10x or 20x objective
>lens. I put the penny at a distance of about 1m or so - it doesn't
>matter, as long as the area around the penny is evenly illuminated.
>The shadow is about 2m from that, although the exact distance doesn't
>seem to matter much if you're just looking for the existence of a
>spot. I've only used it in small classes, so I've never spent much
>time fine-tuning things for optimal viewing in a large lecture.
>Are we talking about the same thing? I'm talking about the bright
>spot at the center of the shadow of a circular object. All this talk
>of exact lengths worries me - I've never had a problem showing the

>Dr. Eric Ayars
>Assistant Professor of Physics
>Walla Walla College
>(509) 527-2476