Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: lyner on 06/01/2008 16:19:35

Title: Umbrellas and diffraction
Post by: lyner 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.
Title: Umbrellas and diffraction
Post by: RD on 14/01/2008 13:52:28
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
Title: Umbrellas and diffraction
Post by: lightarrow 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.
Title: Umbrellas and diffraction
Post by: lyner 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.
Title: Umbrellas and diffraction
Post by: lightarrow 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!
Title: Umbrellas and diffraction
Post by: lyner 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.