How Do Rainbows Form—and What Causes a Double Rainbow?

Rainbows seem almost magical, but they’re really a beautiful demonstration of what happens when sunlight meets water.

You don’t actually need a rainstorm to make one. Tiny droplets from mist, a waterfall, a garden sprinkler or even a hose can produce the same effect—as long as the sun and water are positioned just right.

How Does a Rainbow Form?

A rainbow forms when sunlight enters tiny water droplets suspended in the air.

Although sunlight looks white, it contains many wavelengths of visible light, which we see as different colors. When sunlight passes from air into a raindrop, the light slows slightly and bends. This bending is called refraction.

Different wavelengths bend by slightly different amounts. Violet light bends more than red light, so the colors begin to separate.

But that’s only the first part of the journey.

Once inside the raindrop, some of the light reflects off the back of the drop. As it leaves the drop and passes back into the air, it bends again. By the time the light reaches your eyes, the colors have spread apart enough for you to see them individually.

That combination of refraction, reflection and refraction again creates the familiar band of rainbow colors.

The droplets producing a rainbow are also constantly falling and changing as they move through the atmosphere. A raindrop can have a surprisingly complicated journey before it ever reaches the ground, as we explain in How Far Does a Raindrop Fall—and What Happens on the Way Down?.

Why Are Rainbow Colors Always in the Same Order?

In a primary rainbow, the colors always appear in the same order because each wavelength of light bends by a predictable amount.

From the outside edge of the arc toward the inside, the order is:

Red, orange, yellow, green, blue, indigo and violet.

You may remember it as ROYGBIV.

Red light is bent the least, so it appears on the outer edge of the primary rainbow. Violet is bent more and appears along the inner edge.

The colors aren't really seven separate stripes, though. A rainbow contains a continuous spectrum of visible light, with one color gradually blending into the next.

Why Is a Rainbow Curved?

A rainbow isn't a physical object hanging in one particular spot in the sky. What you see depends on the angle between the sun, the water droplets and your eyes.

The brightest light in a primary rainbow reaches you from droplets at an angle of roughly 42 degrees from the point directly opposite the sun.

Because all the droplets sending rainbow light toward your eyes at that angle form a circle around that point, a rainbow is actually circular.

From the ground, the horizon usually blocks the lower part of the circle, leaving us with the familiar arc. From an airplane or another high vantage point, it's sometimes possible to see nearly an entire circular rainbow.

Why Does the Sun Have to Be Behind You?

To see a rainbow, the sun needs to be behind you and the water droplets need to be in front of you.

That's why rainbows are often especially noticeable when a rain shower is moving away while the sun breaks through behind you. It's also why sunshowers—when rain falls while the sun is shining—can provide ideal conditions for seeing a rainbow.

The sun's position also affects how high the rainbow appears. When the sun is low in the sky, the rainbow can appear high and dramatic. When the sun is higher, the visible arc sits lower.

Once the sun rises more than about 42 degrees above the horizon, a normal primary rainbow is generally below the horizon and can't be seen from ground level.

That's one reason rainbows are more common in the early morning and late afternoon than around midday.

Why Are There Sometimes Two Rainbows?

Sometimes conditions are right for a double rainbow.

The brighter inner arc is the primary rainbow. The fainter outer arc is called the secondary rainbow.

A secondary rainbow forms because some sunlight reflects twice inside each raindrop before leaving it, rather than reflecting once.

That extra reflection makes the secondary rainbow dimmer, but it also produces one of the most interesting features of a double rainbow: the colors are reversed.

In the primary rainbow, red is on the outside and violet is on the inside. In the secondary rainbow, violet is on the outside and red is on the inside.

So the two red bands face each other.

Why Is the Sky Darker Between Two Rainbows?

If you look carefully at a strong double rainbow, you may notice that the sky between the two arcs looks darker than the sky beneath the primary rainbow.

This darker region is known as Alexander's band, named after Alexander of Aphrodisias, who described the phenomenon nearly 2,000 years ago.

It happens because relatively little rainbow light is directed toward your eyes from the region between the primary and secondary arcs. The contrast makes the band look noticeably darker, especially when the rainbows are bright.

Can There Be More Than Two Rainbows?

In theory, yes.

Light can reflect three, four or even more times inside water droplets, producing higher-order rainbows. But each reflection loses some light, making these rainbows extremely faint and difficult to see.

Third- and fourth-order rainbows are especially challenging because they appear in a different part of the sky from the familiar primary and secondary arcs and can be overwhelmed by sunlight.

For most of us, spotting a good double rainbow is rare enough.

Why Can You See Rainbows in Waterfalls and Sprinklers?

Raindrops aren't special when it comes to making rainbows. What matters is having many small water droplets in the air and sunlight hitting them at the right angle.

That's why you can sometimes see miniature rainbows in:

• Waterfall mist
• Ocean spray
• Fountains
• Lawn sprinklers, garden hoses and fine mist from irrigation systems

The physics is essentially the same as it is in a rainstorm.

On a sunny day, try standing with the sun behind you while spraying a fine mist of water into the air in front of you. Move the spray—or yourself—until the angle is right, and a small rainbow should appear.

Is Everyone Looking at the Same Rainbow?

Surprisingly, no.

The rainbow you see is created by light reaching your eyes from particular droplets at particular angles. Someone standing a few feet away receives light from a slightly different collection of droplets.

So although two people can certainly look at what appears to be the same rainbow, each observer is technically seeing their own. And because raindrops are constantly falling and being replaced, even your own rainbow isn't being produced by the exact same droplets from one moment to the next.

A rainbow may look like a fixed arc stretching across the landscape, but it's really an ever-changing interaction between sunlight, water and the person looking at it.