How do rainbows get their colors in order?
Rainbows always show colors in the same order, red, orange, yellow, green, blue, indigo and violet, because raindrops bend each color of sunlight by a slightly different amount.
When sunlight enters a raindrop, it slows down and bends, a process called refraction. It reflects off the back of the drop and bends again as it leaves. Different colors, or wavelengths, bend by different amounts. Red light bends the least and violet light bends the most, which spreads sunlight into a band of colors.
Each color leaves raindrops at a slightly different angle, around 42 degrees for red and 40 degrees for violet. So the color you see from each drop depends on its position in the sky. Drops higher in the sky send red light to your eyes, and lower drops send violet, creating the familiar order with red on the outside.
Double rainbows
Sometimes a second, fainter rainbow appears outside the first. It is caused by light reflecting twice inside each raindrop, which reverses the colors so red is on the inside. The darker band of sky between the two rainbows is called Alexander's band.
Did You Know?
A rainbow is actually a full circle. From the ground, the horizon usually hides the lower half, but pilots sometimes see complete circular rainbows.