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http://antwrp.gsfc.nasa.gov/apod/ap040913.html]The good people at Astropix have challenged us!
*Okay...the person taking the photo has his head directly in front of the Sun (the "bright area in the center"). The ground has a bit of oil on it -- and water. This creates a circular "rainbow" type of effect. :hm:
???
Proper name for it? Beats me...and I'm certainly not sure my "explanation" is the correct one (except that I -am- sure his head is directly in front of the Sun, blocking it).
--Cindy
We all know [i]those[/i] Venusians: Doing their hair in shock waves, smoking electrical coronas, wearing Van Allen belts and resting their tiny elbows on a Geiger counter...
--John Sladek (The New Apocrypha)
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Okay, now that's interesting.
As for the explanation:
The picture is of the photographer's shadow, with the sun directly behind him/her. The background is concrete - probably a parking lot, but exact location is not important. The rainbow effect is due to diffraction of the sunlight by the photographer's hair.
To see how this works, hold the tips of your thumb and forefinger very lightly together and look between your fingertips while holding them very close to your eye. You have to look so closely that your fingers look fuzzy around the edges. In fact, if you look between them at just the right angle (tangent to both fingertips while touching them together) the "fuzzy" part around their edges will appear to resolve itselve into little regions that float between the tips when you open them a tiny distance apart. This fuzzy region and the little region visible between your fingertips are due to diffraction and interference between diffraction patterns as the light is diffracted around the tiny ridges of your fingerprints. It's not just an optical illusion - the effect is visible in a properly illuminated shadow puppet, too. Only on the scale of a shadow puppet those fuzzy regions will separate enough to resolve into colors.
In the photograph, instead of fingerprint ridges, human hair is diffracting the light. This gives a much more evenly distributed rainbow.
(The photographer's jacket and hands are also refracting a little light, giving his/her shadow a little fuzz around the edges in the picture. But the diffraction isn't sharp enough to separate the colors in the photograph.)
The diffraction isn't much. Most of the light just goes straight through and mixes together as white light on the other side. That's why the center appears brighter, BTW. It is slightly more intensely illuminated, catching both refracted light from the photographers hair and incident sunlight, but you wouldn't notice it if the spectrum of the incident light weren't more evenly mixed and thus "whiter" than the sunlight. (Notice how the top of the circular halo appears dimmer than the part around the shoulders? That's the light that came through the photographer's bangs, which are apparently cut thinner than the poofy feathers on the side.) The angle of illumination is slightly different inside the halo, too, which may allow a little extra light back to the camera, but not enough to notice without the shift in spectrum.
It occurs to me that it may even be possible to say what color the photographer's hair is. Judging from the shadow, It looks like it's either curly and/or unkempt, with a lot of opacity. The red light was most strongly refracted, which wouldn't be the case with a lot of transmission through the hair strands. (Red is the outside ring in the rainbow, rather than the inside.) Based on that, I'm guessing brown to black, probably not red and definitely not blonde.
"We go big, or we don't go." - GCNRevenger
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It wouldnt be the Sun shining through something like the photographers glasses would it. It would explain the defraction at the edges and how the photographers shadow is dead center in the picture. The glasses of the photographer may well be on his hat as he is using the camera and has put them up to see through the lens better.
Chan eil mi aig a bheil ùidh ann an gleidheadh an status quo; Tha mi airson cur às e.
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It wouldnt be the Sun shining through something like the photographers glasses would it. It would explain the defraction at the edges and how the photographers shadow is dead center in the picture.
Ah, but no appreciable light would be refracted by the rims of the glasses, which would cast a clear shadow just like the rest of the photographer.
Where are the glasses? They should be visible.
"We go big, or we don't go." - GCNRevenger
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The APOD people seem to think the colors are caused by refraction in a special background surfacing material. This is also a possibility.
"We go big, or we don't go." - GCNRevenger
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The body heat from the photographer's head might heat the air around it. This would cause a different air density which would cause the light to refract. Of course, each wave length of light refracts at a different angle, causing the rainbow effect.
My guess, anyway.
"Run for it? Running's not a plan! Running's what you do, once a plan fails!" -Earl Bassett
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http://space.mit.edu/~lewin/apod/]If rain drops were made of glass
*Here's the answer. "Glass bow."
--Cindy
We all know [i]those[/i] Venusians: Doing their hair in shock waves, smoking electrical coronas, wearing Van Allen belts and resting their tiny elbows on a Geiger counter...
--John Sladek (The New Apocrypha)
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