Friday, January 30, 2009

Double Rainbow

We had some rain showers this morning. I happened to glance out the bathroom window as I was shaving and noticed a double rainbow. I raced out onto the back deck to take a photo of this rare event. The primary inner rainbow actually stretched in an unbroken arch right across the sky. It was very impressive.

A double rainbow

The fainter rainbow on the outside is known as a secondary rainbow. Secondary rainbows are caused by a double reflection of sunlight inside the raindrops. As a result of the second reflection, the colours of a secondary rainbow are inverted compared to the primary bow. Note that the blue is on the outside with the red on the inside, opposite to that of the primary bow.

Incidentally, the dark area of unlit sky lying between the primary and secondary bows is called Alexander's band, after Alexander of Aphrodisias who first described it.

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