Date: Thu, 21 Mar 2002 15:59:20 -0500

Author: Norman Derby

Subject: Re: POISSON'S (ARAGO'S) SPOT

Post:

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Look up Babinet 's principle in an optics book or
http://scienceworld.wolfram.com/physics/BabinetsPrinciple.html

Norm Derby


At 11:16 AM 3/21/2002 -0700, you wrote:
> The setup I described creates a dark spot in the middle of an
> illuminated circle of light. My understanding is that it's the same
> physics that predicts a bright spot in the shadow of an object. In the
> former case destructive interference at the center creates a dark spot,
> in the latter case constructive interference at the center creates a
> bright spot...
>
> Jerry
>
>
>At 3/21/2002 12:03 PM, you wrote:
>>I thought and thought about this, and I can't picture it. Why in the
>>world would a small circular aperature throw a dark spot on the wall?
>>
>>Are you puttin' us on here, or is there something I'm missing?
>>-St. John
>>Boston University
>>
>>
>>On Thu, 21 Mar 2002, Jerry DiMarco wrote:
>>
>> > Another neat demo to do is the inverse of Poisson's spot, a dark spot
>> > in the middle of a bright area. I can't find this one in the DCS although
>> > 6C20.30 - Airy diffraction rings seems similar. The Pasco optics kit also
>> > contains a variable aperture that works well for this demo. Just put
>> it in
>> > front of an expanded laser beam and gradually close the iris. You can
>> > watch the diffraction pattern developing, then at minimum aperture a dark
>> > spot appears in the center. It is especially nice and sharp with a
>> > spatially filtered beam...
>> >
>> > Jerry
>

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Look up Babinet 's principle in an optics book or

http://scienceworld.wolfram.com/physics/BabinetsPrinciple.html


Norm Derby




At 11:16 AM 3/21/2002 -0700, you wrote:

     The setup
I described creates a dark spot in the middle of an illuminated circle of
light.  My understanding is that it's the same physics that predicts
a bright spot in the shadow of an object.  In the former case
destructive interference at the center creates a dark spot, in the latter
case constructive interference at the center creates a bright
spot...


                                                               
Jerry




At 3/21/2002 12:03 PM, you wrote:

I thought and thought about this,
and I can't picture it.  Why in the

world would a small circular aperature throw a dark spot on the
wall?


Are you puttin' us on here, or is there something I'm missing?

-St. John

Boston University




On Thu, 21 Mar 2002, Jerry DiMarco wrote:


>      Another neat demo to do is the inverse
of Poisson's spot, a dark spot

> in the middle of a bright area.  I can't find this one in the
DCS although

> 6C20.30 - Airy diffraction rings seems similar.  The Pasco
optics kit also

> contains a variable aperture that works well for this demo. 
Just put it in

> front of an expanded laser beam and gradually close the iris. 
You can

> watch the diffraction pattern developing, then at minimum aperture a
dark

> spot appears in the center.  It is especially nice and sharp
with a

> spatially filtered beam...

>

>                                                                 
Jerry




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