What is a double rainbow and how does it form?

In one line

The rule

A double rainbow forms when sunlight is both refracted (bent) and reflected twice inside raindrops, creating two separate arcs of colors in the sky.

There is no single formula, but the main physics involved are:

• Refraction (bending of light)
• Reflection (bouncing of light)
• Dispersion (spreading of colors)

Let's understand it simply

Imagine you are playing cricket in the rain, and the sun comes out. You see a bright rainbow. Sometimes, if you look carefully, you might see a second, fainter rainbow above the first. This is a double rainbow.

Think of each raindrop like a tiny glass ball. When sunlight enters a raindrop, it bends (refracts), bounces inside (reflects), and bends again as it comes out. For a double rainbow, the light bounces twice inside the drop before coming out. This second bounce sends the light out at a different angle, making a second, dimmer rainbow.

How it works

1. Sunlight enters a raindrop and bends (refracts), splitting into different colors.
2. The light reflects off the inside surface of the drop once (for the main rainbow).
3. For a double rainbow, some light reflects a second time inside the drop.
4. After the second reflection, the light bends again as it exits the drop.
5. This double-reflected light comes out at a wider angle (about 50–53° from the sun, compared to 40–42° for the main rainbow).
6. The second rainbow appears above the first and is fainter, with its colors reversed (red inside, violet outside).

The story behind it

The study of rainbows goes back to ancient times, but the double rainbow was first explained in detail by René Descartes, a French scientist, in the 17th century. He used glass spheres and sunlight to show how light bends and reflects inside drops. Later, Isaac Newton explained how white light splits into colors. The double rainbow was a puzzle until scientists realized it was caused by two reflections inside each raindrop.

Where you see it in real life

• After a heavy rain with the sun shining: Look opposite the sun; sometimes you see two rainbows.
• Over fields or lakes during monsoon: The open sky and water droplets make double rainbows easier to spot.
• From a moving train or bus during rain: The changing view can reveal both arcs.
• In photos shared on social media: People often post double rainbow pictures after storms.
• Near waterfalls: The spray creates many droplets, sometimes showing double rainbows.
• In the village during early morning rain: The low sun angle helps form double rainbows.

Why it matters

• Shows how light behaves: Double rainbows help us understand refraction, reflection, and dispersion.
• Used in physics and optics: The same principles are used in making lenses, glasses, and even in fiber optics.
• Inspires art and culture: Double rainbows are seen as lucky or magical in many cultures.
• Helps in weather prediction: Seeing a rainbow means rain and sun are both present, which can hint at changing weather.

Common mistakes

• Thinking the second rainbow is a reflection in water: It's not; both rainbows are in the sky, formed by light inside raindrops.
• Missing the reversed colors: The second rainbow has red on the inside and violet on the outside, opposite to the main rainbow.
• Believing double rainbows are rare everywhere: They are common if you look carefully when the sun is low and rain is light.
• Assuming both rainbows are equally bright: The second rainbow is always fainter because more light escapes after the first reflection.

Remember it

A double rainbow happens when sunlight bounces twice inside raindrops, making a second, fainter, reversed-color arc above the first.