A double rainbow is caused by sunlight reflecting twice inside a raindrop. The first reflection creates the primary rainbow, and the second reflection creates the secondary rainbow, which appears outside the primary arc and has its colors reversed. The secondary rainbow is fainter because more light escapes after two reflections than after one, and it appears about 10 degrees above the primary rainbow. The dark area between the two rainbows is known as Alexander's band, where no light is scattered toward the observer due to the angles of reflection.
How Double Rainbows Form
- Sunlight enters a raindrop and is refracted (bent) at the surface.
- Inside the drop, the light reflects off the back once (primary rainbow).
- For a double rainbow, light reflects twice inside the droplet before exiting.
- The second reflection reverses the color order of the secondary rainbow.
- The secondary rainbow is wider and dimmer than the primary rainbow.
Color Order and Appearance
- The primary rainbow has red on the outer edge and violet on the inner edge.
- The secondary rainbow has the color order reversed, with red on the inner edge and violet on the outer edge.
Visual Effects
- The secondary rainbow appears about 10 degrees outside and above the primary rainbow.
- The dark band between the two rainbows is called Alexander's band, caused by the lack of scattered light between deviation angles of the two rainbows.
Double rainbows commonly occur when the sun is low in the sky, such as in the early morning or late afternoon, and in conditions where raindrops are abundant and sunlight is bright and clear.
