Saturday, August 18, 2007

Somewhere Over the Rainbow

A beautiful double rainbow, and Alexander's Band.

Here is a photo I took this afternoon of a spectacular double rainbow stretching across the sky. We currently are in a pattern of hot days which agitate the atmosphere to create afternoon thunderstorms. Out of this has come some very nice rainbows. I wish I had gone to get past the power lines in these shots, but I didn't want to get my camera wet, as it was raining pretty good at this time.

This particular photo of a double rainbow displays a couple of very cool things about these displays of color in the sky. First, the primary, brightest rainbow is mirrored by a fainter partner, higher in the sky. This is because the light is reflected once inside the raindrops to product the primary, and then is reflected again inside the drops to produce a secondary rainbow, fainter than the original. It is also interesting that the colors are are always reversed in the secondary rainbow.

Also, notice that the sky beneath the primary bow is brighter than that above it. The rainbow becomes a demarcation arc between darker and lighter sky. This is due to the angles of light emerging from the raindrops as well. Since the colors of the rainbow spectrum combine to form white light, there is more white light refracted below the bow than above. The area between the primary and secondary bow is actually a bit darker than the sky above the secondary one. This is known as Alexander's Band. This is because Alexander of Aphrodisias, a commentator on the writings of Aristotle, noted the phenomenon around the year 200 C.E.

While the science behind them is interesting, it doesn't remove any of the joy of looking at the colorful display in the sky.

The primary rainbow's north end, demarcation between light & dark.

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