Date: Mon, 21 Oct 2002 18:08:29 -0400

Author: "Jason St. John"

Subject: Re: light the match demo


Mine are parabolic, too. I know that a parabolic reflector focuses
radiation from infinity at the focus, but if your source is closer than
infinity, the radiation will focus slightly nearer to the dish. In fact,
when I first found my set, I used the heat IR source near a single dish,
with the match a few feet away (the equal and opposite optical path).

So if your students know their geometry, you can do the demo at half the

Jason St. John 617.353.2634
Boston University Physics Lecture Demonstrations

On 2002-10-21.12:56 sent:

I wonted to make another set a few year ago. I found some in Edmund
Industrial Optics item#53-875. I then had our shop make the stands. They
work well but I found that they have two focal points. It is possible that
our shop did something in the mounts that made this occur, Don't know.


At 12:25 PM 10/21/02 -0500, you wrote:
>I shared the handouts from this summers demonstration workshop with some
>other physics instructors and they immediately wanted to do several demos
>(two of which we already had) but we don't have the reflective mirrors to
>do PIRA 4B40.10 the light the match demo. Two curved mirrors (circular?
>parabolic?) one with a filament at its focus the other with a match. Turn
>on the current to the filament/reflector located on one side of the demo
>table and the match in the other reflector located at a distance is
>ignited. I recall that this one was discussed at the workshop but do not
>have any source for the reflectors. Does anyone have a source of some
>appropriate reflectors?
>Don Mathieson
>Tulsa Community College

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