Joel Wilcox wrote:
>I've added another image with sun stars to the page I posted earlier:
>
>http://soli.inav.net/~jdub/sunstar.html
>
>The new image is the first one on the page. This is a sunset image in
>which there are two sets of sun stars created by the sun itself. I didn't
>take the photo with any notion that I would get sun stars. Rather I was
>hoping to catch the beams breaking through the clouds. I got a bit of that
>and a bonus with the sun stars, I think.
>
>Two new questions:
>
>1. Are the beams that I was *trying* to photograph (in the sunset shot just
>added at the link above) also a result of diffraction of any sort?
Nice pictures Joel! Nope, these are called crepuscular rays and are
caused by light scattering off of small particles in the air. The rays
are formed by the structure of the object itself, in your case the
clouds. Sometimes you can also see anti-crepuscular rays, which appear
to converge on the point opposite the sun. You can see some other
examples at
http://www.capetownskies.com/sunbeams.htm
For a book with many pictures and descriptions of various atmospheric
optical phenomena, see Robert Greenler's _Rainbows, Halos and Glories_.
It gets a bit technical, but it's got some great photos in it, especially
of halos caused by ice crystals.
Mark Marr-Lyon
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