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Re: Double concave mirrors. ( Was Writing Lectures)



On Mon, 06 May 2002 13:38:14 -0700 Michael Bowen
<fizzbowen@MINDSPRING.COM> writes:

Dana Andrews <danaandrews2@YAHOO.COM> wrote:
I like to start with a thought provoking demo that sets up the
lesson. example: Using the double concave mirror toy to
demonstrate a
virtual image.
*********************************
Why do you use a DOUBLE concave mirror?
Won't a single concave mirror do the job?

Herb Gottlieb from New York City
Where it is almost impossible to find a double concave mirror in our
science supply catalogs.

*********************************
Michael Bowen Replies>
The double concave mirror can be set up so that the common axis of
the mirrors is vertical. The available demos usually place a small (D
~
5 cm) hole in the center of the upper mirror. The focal lengths and
intra-mirror separation are set so that the edge of the upper mirror
rests
directly on the edge of the lower mirror; when an object is placed at
the vertex
of the lower mirror (that is, resting directly on its surface), the
resulting image makes the object appear to float in mid-air just above
the
central hole in the upper mirror. It's quite startling to see the
image
(which is usually a pretty good copy of the object) floating there, and
even
more startling (to the uninitiated, at least) when they try to touch
the
image with their hand and find only empty air. It's really much more
fun
than merely seeing one's reflection in a single mirror. I'd tell you
where to get one, but I don't have any catalogs handy at the moment.

********************************************
You are referring to a device called "The Mirage". It is listed in the
catalog
of "Educational Innovations" www.teachersource.com Phone 1 888 912
7474
Item #MIR-100 Cost $28.75

It looks like a double concave mirror but it is really two nearby
concave mirrors
with the mirrored surfaces facing each other.

Herb Gottlieb from New York City
( Where the Mirage is one of our favorite physics demo apparatus)