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How does a double rainbow form?

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Quick answer

Sunlight entering a raindrop reflects off its inner surface and bends as it exits, forming the primary bow. Some rays reflect a second time inside the drop, leaving at a steeper angle. Those rays create a fainter secondary arc about 10 degrees higher, with reversed colours and a dark band between the two.

A double rainbow forms when sunlight is reflected twice inside the same raindrop before it exits, sending light back at a higher angle than the primary bow. The second reflection reverses the colour order and spreads the light more thinly, which is why the secondary arc looks fainter.

Step by step

  1. Sunlight strikes a falling raindrop and bends as it passes from air into water.
  2. The light reflects off the inside back surface of the drop and heads back toward the front.
  3. As it exits the drop, it bends again and splits into colours, sending the primary bow toward your eye.
  4. A fraction of the light reflects a second time inside the drop instead of leaving.
  5. That second bounce pushes the outgoing light to a wider angle, so the secondary bow forms higher in the sky.
  6. The extra bounce reverses the colour order and dims the light, giving the secondary bow its fainter look.

Key things to understand

  1. Sunlight enters a raindrop, bends, bounces off the inside back wall and bends again as it leaves, creating the primary rainbow.
  2. Some light bounces a second time inside the drop before escaping, and each extra bounce widens the angle between incoming and outgoing light.
  3. Because of that wider angle, the secondary bow appears about 10 degrees above the primary one, with darker sky between them.
  4. Every reflection flips the light path, so the secondary rainbow shows red on the inside and violet on the outside, the reverse of the primary.
  5. Each bounce also scatters and spreads the light, so the secondary bow carries less intensity and usually looks noticeably fainter.

Labelled diagram

  1. Sunlight enters

    White light hits a raindrop and bends as it crosses into the water.

  2. First reflection

    Light bounces off the inside back wall of the drop.

  3. Primary exit

    Light bends again leaving the drop, splitting into colours that form the main bow.

  4. Second reflection

    Some light bounces a second time inside the drop before escaping.

  5. Secondary exit

    The extra bounce widens the angle and reverses the colour order, making a fainter higher bow.

How a double rainbow forms in raindrops

Real-world examples

  • From a hillside after a summer storm, a bright primary bow often sits below a faint secondary arc.
  • Photographers at Niagara Falls frequently capture double rainbows in the mist rising from the falls.
  • In Hawaii, early-morning showers with the Sun low behind you often produce a clear double rainbow over the ocean.
  • A garden hose sprayed away from the Sun on a sunny day can create a small double rainbow in the droplets.

Common mistake

✗ Myth: A double rainbow means two separate rainbows formed by two separate rainstorms.

✓ Fact: Both bows come from the same sunlight and the same raindrops. The difference is how many times the light reflects inside each drop before it exits.

People also ask

What causes a double rainbow?
A double rainbow appears when some sunlight reflects twice inside raindrops instead of once. The first reflection makes the primary bow; the second reflection sends light out at a wider angle, creating a second, fainter arc above it. Both bows need the Sun behind you and rain in front of you.
Why is the second rainbow fainter?
Each reflection inside a raindrop loses some light and spreads the rest over a wider angle. By the time light escapes after two bounces, much less of it reaches your eye. That is why the secondary bow looks pale compared with the primary one.
Why are the colors reversed in a double rainbow?
Every reflection flips the path of the light inside the drop. After two reflections, red leaves at a different angle than it did after one, so red ends up on the inside of the secondary bow. Violet then sits on the outside, the opposite of the primary rainbow.
Why is the sky between the two rainbows darker?
That band is called Alexander's band, named after an ancient Greek observer who described it. In that zone, light from the primary bow's angles and the secondary bow's angles does not reach your eye, so fewer raindrops send light your way. The result is a dimmer strip separating the two arcs.
Can you ever see a third or fourth rainbow?
Yes, but very rarely. Third and fourth bows form on the same side of the sky as the Sun, roughly 40 degrees from it, where bright sky swamps their faint light. They were first photographed in nature only in 2011.
Do double rainbows always appear together?
No. The secondary bow needs the same geometry as the primary, so it is visible only when the Sun is low and rain is falling in front of you. If the Sun climbs too high, both bows drop below the horizon. The secondary is fainter, so haze or cloud can hide it while the primary stays visible.
Why do rainbows always curve?
The curve is an illusion of perspective, not a physical arch. All raindrops that send light to your eye at the rainbow angle lie on a cone pointing away from the Sun, and your eye sees that cone as a circle. From the ground, the horizon cuts the circle, leaving an arc.

Part of the guide: Water & Weather Phenomena →