🚀 go-pugleaf

About Larry's observation of the two shadows in the text below:

I could try to launch into a theoretical discussion of extended sources
in optics but I won't.  Instead, wouldn't it be interesting if the
students could construct some of their own explanations with your help.
Suppose for example they were to simulate the eclipse with a cardboard
disk and a lightbulb or flashlight and observe the shadows made by a
pencil.  Unlike the real eclipse, here they can control the variables
and the phenomena.

You might simulate simpler situations to build conceptual
understanding of the optics.  For instance, what happens if the sun were
eclipsed so that what was exposed were a straight slit?  Does the
sharpness of the shadow depend on how wide the slit is?  What does the
the relative orientation of the pencil and the slit - perpencicular or
parallel- do to the shadow?

Now, what happens if there are two slits?

Do any of these simulations along the way resemble the real eclipse?


Those were just some thoughts and guiding questions with the caveat that
I have not tried any of this out.  My gut feeling from having done a lot
of optics in the past though is that you might find some out some
interesting things to help understand what you observed.

Actually I had a rather disappointing time trying to get people here
excited about using two index cards, one with a hole in it like a
pinhole camera, the other a screen in the shadow of the first.  People
prefered to view the eclipse directly through various filters.  I think
it is so neat that something as simple as a pinhole and screen can make
an image.

But if you think of the image of the eclipse on the lower card as the
anti-shadow of the pinhole, then maybe you can connect the shadow effect
Larry observed with the shape of the sun-moon image.  (The image is the
anti-shadow, because the image is the where the light gets through and
everywhere else it is dark, whereas a shadow is just the opposite.)
Maybe even go back to the simulation and make the pinhole a slit - where
you think of the slit as an anti-pencil!

Sorry, went off the deep end.

Bill Barowy

>
>Viewed today's annular eclipse here in mid Missouri.....always gives me a
>thrill to teach about the pinhole camera and be able to demonstrate on
>such a grand scale.  About 1/2 through the eclipse the class noticed a
>strange phenomena.....shadows, normally sharp in the sunlight, became
>two....that is two shadows one darker than the other occured with say a
>pencil ...... one explanation--each part of the sun still visible was
>casting its own shadow...but there was no discontinuity in the sun-just a
>half ring sorta...again(for the gazillionth time in 30 yrs teaching
>physics) I admitted I didn't have an explanation.
>Larry

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Bill Barowy
Educational Technology; Bolt Beranek and Newman,
150 CambridgePark Drive, Cambridge, MA O2140
(Work) 617-873-2036  (Fax) 617-873-2455