Dear Tom,
Why do the colors reverse between a rainbow and secondary rainbow (when a secondary appears)?
—Janice Mercer, Bolingbrook
Dear Janice,
It’s a beautiful sight: a rainbow and a secondary rainbow at a greater angle above it. The term “double rainbow” is used when both the primary and secondary rainbows are visible. A primary rainbow is the result of the refraction and reflection of light. Light entering a water droplet is refracted and then reflected from the back of the droplet. As this reflected light leaves the droplet, it is again refracted. A secondary rainbow is caused by a double reflection of sunlight inside the droplet. The “inside” of the secondary is “up” to the observer and its colors appear reversed compared to the primary. A search of “double rainbow formation diagrams” provides visual explanation.