Re: Writing Lectures

[Michael Bowen <fizzbowen@MINDSPRING.COM>]

At 15:01 2002/05/06, Herbert H Gottlieb wrote:
> On Tue, 30 Apr 2002 06:13:38 -0700 Dana Andrews <danaandrews2@YAHOO.COM>
> writes:
> > 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 gthe job?
>
> Herb Gottlieb from New York City
> Where it is almost impossible to find a double concave mirror in our
> science supply catalogs.

Herb,

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.

--MB