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Re: [Phys-l] Interesting Shadows



Hello,
Delighted to hear that others are thinking about similar things as myself. We recently put together some laboratory activities for physical science teachers about shadows. It is all about the penumbra and umbra, the finite size of the light source, the size of the object or holes producing the shadows. the screen where the shadow is observed, and the distances between the source, object, and screen and the fact that light rays travels in straight lines. At first other faculty thought these activities were too simple, but after teaching these f activities to teachers, it feel now it was right on target and within the purview of what teachers should be teaching at the physical science level.
Richard

On Feb 22, 2011, at 6:24 PM, Roger Haar wrote:

Hi,

There are things I cannot tell from your
photo's so I could easily be wrong. I suspect it
involves the umbra and the penumbra of the shadow.
When something is illuminated by an extended light
source the object cast a shadow with an umbra
and a penumbra. The area which is the umbra is
darkest and is not illuminated at all by the source.
The area which is the penumbra is illuminated by
only part of the extended source and varies from
light to dark.

As the "screen" on which the shadow is
cast gets further from the object, the penumbra
becomes larger but the umbra gets smaller and
eventually vanishes.

This is easy enough to see if one does
a bit of ray tracing. Try this with an extended
source (rays diverging from each point of the
source) and a "object" that is two obstructions,
like:


s
o | S
u C
r | R
c E
e E
N
Stretch this out as much as you can. At some point the
penumbra of the two obstructions overlap and darken the
shadow between their individual shadows.

Thanks
Roger

On 2/22/2011 2:39 PM, Marc "Zeke" Kossover wrote:

Howdy-

My school (The Jewish Community High School) has vinyl letters of it's name on a
window, and on sunny mornings they cast a shadow on the wall across the
hallway.

If you imagine the shadow made by the letters, you might picture a shadow of
each of the letters with blurry edges. You might imagine the centers of the
letters to be bright, but they aren't.

The weird bit is that the darkest parts on the wall are where the holes in the
letters would be. They are even darker than the shadows of the letters.

You can see pictures at my infrequently updated blog.

<http://kossover.squarespace.com/journal/2011/2/11/weird- shadows.html>

I presume that it's diffraction related. What say you?

Marc "Zeke" Kossover



_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

Dr. Richard A. Lindgren
Research Professor of Physics
Department of Physics
382 McCormick Rd.
University of Virginia
Charlottesville, VA 22904-4714 USA
ral5q@virginia.edu
Office 434-982-2691
Fax 434-924-4576