Re: [Phys-l] Interesting Shadows

[Roger Haar <haar@physics.arizona.edu>]

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
>
>
>
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