Backwards second rainbow

Why is the second rainbow backwards? —Teddy, Albuquerque, New Mexico

The second bow shines with backwards beauty Photo courtesy of NOAA.

The second rainbow arches above the first—its red faces the red of the first. The second bow’s colors are "backwards" compared to the first as seen in the figure.

The second bow reverses colors because each sunray enters a raindrop at the bottom (instead of the top of the raindrop) and bounces an extra time (twice) inside the droplet. The figure by Rod Nave shows what happens.

I’m indebted to Rod Nave for this illuminating figure. [Rod Nave, Georgia State University]

Nave’s figure is worth a thousand words in trying to explain why these differences cause red to bend more and therefore ends up lower in the resulting secondary bow. See his figure for the explanation.

Since red bends more than violet, it will likely be too high to hit the viewer's eye—unless, the raindrop is low in the sky.  Consequently, red appears low in the secondary bow.  Likewise, the opposite occurs for a primary bow since red bends less (42°, compared with 40° for violet). See the figure for how red ends up at the top of the primary bow. Thus, the reds of the two bows end up together and the colors throughout are reversed.

The second bow is almost always visible. Though faintly. It’s only one tenth as intense as the primary and almost twice as broad. It’s weaker because less light survives the longer trip in the raindrop and the extra bend.

Further Surfing:

Look down on a rainbow to see a whole circle, WeatherQuesting

Why the second rainbow colors are backwards, WeatherQuesting

How rainbows form, WeatherQuesting

What a rainbow looks like to a dinosaur, WonderQuest

Glory (circle) rainbows seen from a plane, WeatherQuesting

Why the inside of a rainbow is bright, WonderQuest

Rod Nave, Hyperphysics: Rainbows

Weather Basics: Rainbows

(Answered Aug. 15, 2003, updated July 20, 2007)

New Comment --- add your comments to the discussion