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On a dry, sunny day this summer, I was walking along my street when, looking down, I noticed a rainbow halo centered around the shadow of my head. I posted photos to Twitter, asking for help explaining its appearance. Through a mutual friend, Dr. Shane and I connected and, after some exchanges and further experimentation (such as wiping away some of the dirt in which the rainbow seemed to appear, which made that section of the rainbow disappear), we concluded that there were probably some kind of small transparent spheres mixed in with the dirt, which served the same purpose as water droplets in the creation of a rainbow.

I mailed Dr. Shane a sample of this dirt, as well as of some “non-rainbow dirt” from down the road, which he examined under a microscope. Indeed, he observed...

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