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Author Topic: Umbrellas and diffraction  (Read 2667 times)

lyner

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Umbrellas and diffraction
« on: 06/01/2008 16:19:35 »
I have always been fascinated by the patterns you can see when you look at distant street lamps through a dark coloured umbrella fabric, particularly at night, of course. You will notice a 'star' shape of dots spreading out from a central image of each lamp. The pattern stays still as you move the umbrella from side to side but rotates as you rotate the fabric. I couldn't believe that diffraction would be visible from such a coarse structure but the sums agree. The best effect is when you view the old fashioned sodium lamps because of their narrow spectrum.


 

Offline RD

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Offline lightarrow

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Umbrellas and diffraction
« Reply #2 on: 14/01/2008 14:48:54 »
A laser shone through fabric also produces this effect ...

http://www.sciencephoto.com/images/imagePopUpDetails.html?pop=1&id=652050104&pviewid=&country=67&search=diffraction+AND+cloth&matchtype=FUZZY
Infact a sodium lamp (~ monochromatic) at high distance is approximately a coherent source.
 

lyner

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Umbrellas and diffraction
« Reply #3 on: 15/01/2008 11:14:30 »
I noticed this effect when at School -  long before you could buy lasers and the sexy thing about it is that you don't need any fancy equipment to see a very sophisticated effect ; a candle would do.
 

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Umbrellas and diffraction
« Reply #4 on: 15/01/2008 11:30:33 »
I noticed this effect when at School -  long before you could buy lasers and the sexy thing about it is that you don't need any fancy equipment to see a very sophisticated effect ; a candle would do.
Think about the Michelson-Morley experiment, made with common light sources!
 

lyner

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Umbrellas and diffraction
« Reply #5 on: 15/01/2008 21:51:49 »
Yes. To hear some people talk, you would think that life only began with the invention of the Laser.
 

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Umbrellas and diffraction
« Reply #5 on: 15/01/2008 21:51:49 »

 

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