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Re: A list of textbook misconceptions





On Wed, 11 Feb 1998, Leigh Palmer wrote:

One of my favorite misconceptions is one that is drawn by students on
seeing the standard picture of a ruler or pencil inclined in a glass
of water. The pencil looks bent at the surface of the water, and
students often take this to mean that the light bends just like that,
too. (Note that the pencil appears to bend away from the surface
normal where it enters the water.)

Leigh, this is one of my "favorites" also. I particularly like the
textbook pictures, such as those in Hewitt, which show this in an
ungraduated cylinder. If you do this in such a cylinder you find that the
pencil "bending" (and break at the water surface) is due to the
cylindrical lens action of the beaker, as can be demonstrated by putting
the pencil in vertically, and moving it from center to edge. The image
below water is magnified by the curved sides of the glass.

I also like Hewitt's cartoon of the prof doing this demo (again in the
cylinder), and pulling out the pencil and it's really bent! I cut a wooden
pencil, cut the exposed edge and glued it back together so it is bent. I
have another unaltered pencil just like it in my pocked. If you hold the
pencil at the break, with the bend toward you, it looks straight to
students (stand at the corner of a square classroom). Then put it straight
down in the water, and as you tilt it, rotate it simultaneously in your
fingers 90 degrees. It looks to students just like Hewitt's picture. Then
pull out the pencil. Then show what it looks like with the real pencil
(not so dramatic, is it?) Then explain it all.

Hewitt doesn't mention the effect of the beaker as a cylindrical lens. He
passes up an opportunity to really explore some concepts here.

-- Donald

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Dr. Donald E. Simanek Office: 717-893-2079
Professor of Physics FAX: 717-893-2048
Lock Haven University, Lock Haven, PA. 17745
dsimanek@eagle.lhup.edu http://www.lhup.edu/~dsimanek
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