Comments and questions about the APOD on the main view screen.


Post by NotAGlory » Thu Sep 16, 2004 6:50 am

This was posted by "Emma" it can be found on page 5 of the original Levin thread, it has been confirmed by Dr.Levin

This is a the same as the physics occuring when a rainbow is formed. The ground must have had some kind of grains in it that were spherical in shape which light can pass through.

Consider just the top of the rainbow pattern. The light refracts once upon entering the grain and the blue light is refracted through a greater angle than the red light as it has a larger index of refraction. The seperated light then reflects of the back of grain and the blue light is therefore deflected upwards more than the red. The light is the refracted again upon leaving the grain causing an even greater seperation. Because the blue light is upward more, the blue light which reaches the camera comes from grains which are lower down. This happens the entire way around in a circle and explains the colour ordering.

The bright area in the center is also a phenomenon which can be seen in rainbows (there is bright section either side of the primary and secondary rainbow with a dark patch in the center). The light paths for parallel rays through the grain which produce the rainbow pattern come from the grains that are furthest from the center of the circle which have single reflections. All others reach the eye from further in grain which lighten the area. This effect is made even more noticable because any grains further out than the circle can not bend the light towards the camera which makes this region darker than the actual ground.

There should be a 2nd circular rainbow further out on than the photo with the reverse colour order which corresponds to light which is doubly reflected within the grain. Further out from here would be another light patch.

A good website which explains rainbow physics is ... on.html#c1

Dan Cordell
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Joined: Mon Jul 26, 2004 8:55 pm
Location: Michigan Tech

Post by Dan Cordell » Thu Sep 23, 2004 4:34 am

Please keep all discussion related to Dr. Lewin's challenge in this thread.
Dan Cordell, Giant Space Cow