Why can rainbows appear double?
Short answer
A double rainbow appears when sunlight is reflected twice inside raindrops before reaching your eyes. The outer rainbow is called a secondary rainbow, and it is dimmer with its colors in reverse order.
How it works
When sunlight enters a raindrop, it bends and separates into different colors. In the primary rainbow, light reflects once inside the drop before leaving it.
Some light reflects one more time inside the drop. When that light leaves at another angle, it appears as a secondary rainbow outside the primary one. The extra reflection makes it weaker, so the secondary rainbow looks fainter.
Key point
A secondary rainbow is not a reflection of the first rainbow in the sky. It requires the right arrangement of the Sun behind the observer, raindrops ahead, and light traveling into the observer’s eyes.