Grand Canyon Rainbows
Images by James Young (photo gallery) at Yavapai Point on the South Rim, September 11, '11.
©James Young


Atmospheric
Optics

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A rainstorm over the canyon has given an incredibly bright and classical rainbow display. What caught most eyes were the colourful primary (one reflection inside raindrops) and reversed colour secondary (two internal reflections). The bows are concentric and centered on the antisolar point – Camera distortions make the distance between them appear to vary.

More subtle features are the brighter sky inside the primary and outside the secondary. Between the bows there is darkness – Alexanders Dark Band. Not in this case the name of a pop group but after Alexander of Aphrodisias who first recorded it around 200AD.

The coloured rims of rainbows are formed by glints from water drops near the edge of two ‘rainbow cones’ whose common apex is at the eye and whose axes point towards the antisolar point. The inner cone of the primary bow has an apex half angle of 42°.

It is only near the cone surfaces that individual coloured glints overlap sufficiently little that we can see the colours. Raindrops further from the cone surface also glint their coloured light but then there is so much colour overlap that we see instead a white glow. The glow is inside the primary cone and outside the secondary cone.

Raindrops between the two cones cannot glint any light towards the eye. They do glint coloured light but it is directed away to someone else’s rainbow.    Does a rainbow exist when no one is there to see it?