Last Thursday evening my husband and I were treated to a gorgeous double rainbow with parallel color bands under the main arc.  What are these wonders and what causes them?

First, some fascinating basics: Rainbows are caused by light hitting water droplets and being reflected, refracted and dispersed by them:

• Reflected from both the outside face of the water drop and its inside wall.
• Refracted or bent as it enters and exits the raindrop.  Refracted twice!
• and Dispersed from white light into seven colors: red, orange, yellow, green, blue, indigo and violet.

We saw the rainbow at dusk while the sun peeked below rain clouds and were surprised we could see the entire arc. Only those located at the geometric center of the rainbow, the antisolar point, can see it end to end.

Have you ever noticed that the sky under the rainbow is brighter than the sky outside it?  True!  Click here for an explanation (fourth paragraph).

And then there’s the fancy stuff: the double rainbow and the extra bands under the arc.  To illustrate them I’ll use this photo from Alaska that shows all of the features at the same time. Click on the image to open a high definition version in a new window and see them up close.

Double rainbows happen when light bounces twice inside the raindrops before it exits.  The light goes inside, reflects off the back wall, reflects off the front wall, and then exits.  Double rainbows are parallel to the main rainbow, are not as bright, and their colors are reversed — red-to-violet instead of violet-to-red.  I never noticed that. I’ll have to watch for it next time.

The faint color bands just under the main rainbow arc are called supernumerary rainbows.  Wikipedia says they cannot be explained using classical geometric optics but they occur when the water droplets are less than 1 mm in diameter.  Fancy rainbows are complicated!

And finally, the end of the rainbow. From our vantage point it was in front of that tree on the horizon.

Irish legend has it that leprechauns hid their pots of gold at the end of the rainbow. I wonder whose house was there.

Rainbows move away as you approach.  There’s no way to know.

(photos of neighborhood rainbow by Rick & Kate St. John. Alaskan rainbow by Eric Rolph via Wikimedia Commons)