Rainbow Facts: How They Form, Why They Bend, and What Makes Doubles

A rainbow is the most familiar piece of sky optics on the planet, and most readers can name two facts about how it works. Here are seven, plus the geometry, the folklore, and the lesser-known cousins (double rainbow, fogbow, moonbow, supernumerary). Once you know what to look for, the next bow you spot will be three times as interesting.

How a Rainbow Actually Forms

A rainbow is an optical effect produced when sunlight enters a falling water droplet, bends (refracts), bounces off the back inside surface, and exits the droplet bent again on its way out. White sunlight is made of all the colors of the visible spectrum mixed together. Each color refracts by a slightly different amount because of its wavelength, so the white light separates into its component colors as it bends. The droplet acts as a tiny prism, and millions of droplets together produce the visible bow.

The geometry forces all of that bent light to leave the droplet at the same angle, around 42 degrees from the original sunlight direction. That fixed angle is what makes the rainbow a circle (or, when the horizon cuts it off, an arc). Every droplet that sits 42 degrees from the line connecting your eye to the sun’s anti-solar point contributes light to the rainbow you see. Move two steps to the left, and a slightly different set of droplets does the work; the rainbow you now see is technically a different rainbow.

The Seven Facts Worth Knowing

1. Rainbows Always Sit Opposite the Sun

Where you see a rainbow depends on where the sun is and where you are. The bow forms in a circle around the anti-solar point, the spot in the sky directly opposite the sun. If the sun is low in the western sky behind you, the rainbow appears in the eastern sky in front of you. The top of the arc is always centred on the line that runs from the sun through your head to the ground. Two people standing 100 feet apart see two slightly different rainbows, because their lines of sight produce different geometric paths.

2. The Colors Run in a Fixed Order: ROY G BIV

Every rainbow shows the same color sequence in the same order, from red on the outside to violet on the inside: red, orange, yellow, green, blue, indigo, violet. The standard mnemonic for remembering the order is the imaginary man’s name ROY G BIV. The order comes from wavelength: red is the longest visible wavelength and bends the least, violet is the shortest and bends the most.

3. A Double Rainbow Is Not Twice the Reflection, It Is Twice the Bounce

A double rainbow appears when sunlight reflects twice inside the water droplet before exiting, instead of just once. The second internal reflection sends the light out at a wider angle (about 51 degrees instead of 42), so the secondary bow sits outside the primary bow. It is fainter because each internal reflection loses some light, and the colors run in reverse order: violet on the outside, red on the inside. If you see a double rainbow, look for the color flip; that confirms the second bow is real and not just a thicker version of the first.

4. The Sky Inside the Bow Is Brighter

Look closely at any rainbow and the sky inside the arc is noticeably brighter than the sky outside it. The droplets inside the rainbow’s arc reflect a small amount of light at every angle less than 42 degrees, so the sky inside the bow gets an extra boost of scattered light. Outside the bow, no such reflection reaches your eye, and the contrast makes the outer sky look dim by comparison. In a double rainbow, the dark gap between the primary and secondary bows has a name: Alexander’s band, after Alexander of Aphrodisias, the Greek philosopher who described the phenomenon roughly 1,800 years ago.

5. Rainbows Are Actually Full Circles

The arc you usually see is the top half of a circle. The bottom half is hidden by the horizon because there are not enough water droplets at ground level to light up the lower half of the bow. From the ground you almost never see the full circle. From a plane in flight, you can: the rainbow appears as a complete ring around the plane’s shadow, which sits at the anti-solar point in the middle of the circle. Pilots and passengers regularly photograph circular rainbows from above storm cells.

6. There Is No End of the Rainbow

The rainbow is an optical effect, not a physical object. It has no fixed location in space. As you walk toward it, the geometry shifts and the rainbow moves with you. You will never reach the end of one. The Irish folklore of a pot of gold at the rainbow’s end is built on a piece of optics that makes the goal physically impossible. (The folklore is older than the science, of course; the pot of gold predates the geometry.)

7. Rainbows Need Sun in Front, Rain in Back

The basic ingredient list for a rainbow is short: sunlight from behind you, falling water droplets in front of you, and a sun angle of less than about 42 degrees above the horizon. That last condition is why rainbows are most common in early morning and late afternoon and almost never appear at noon in summer. When the sun is too high, the rainbow’s arc dips below the horizon and you cannot see it from the ground. The same rule applies to looking for one: stand with your back to the sun, look at the rain, and scan around the anti-solar point.

The Lesser-Known Cousins

The standard rainbow has a few rarely-seen relatives. Each is produced by the same basic refraction-and-reflection physics, but with different ingredients.

• Fogbow: a colorless or near-colorless arc that appears in fog or low cloud. The droplets are smaller than rain droplets, which smears the colors out into a white or pale band.
• Moonbow: a rainbow produced by moonlight, usually faint and ghostly. Most common around a full moon when there is rain or spray. Famous moonbows are documented at Cumberland Falls in Kentucky and Victoria Falls on the Zambia-Zimbabwe border.
• Supernumerary bow: faint bands of pink and green just inside the primary rainbow, caused by light-wave interference rather than simple refraction. They appear when the droplets are uniform in size.
• Reflection rainbow: a rainbow produced when sunlight reflects off a body of water before refracting through droplets. Looks like a normal rainbow but rooted in a different geometric origin.
• Brocken spectre: not a rainbow exactly, but a related phenomenon. A circular rainbow ring (called a glory) around the shadow of an observer cast on cloud or fog below. Most commonly seen on mountain summits or from aircraft.

For more on the variations, see our piece on the different types of rainbows.

Rainbows in Folklore and Tradition

Rainbows show up in nearly every culture’s mythology. The Old Testament places a rainbow as God’s covenant with Noah after the flood. Greek mythology made the rainbow into Iris, the messenger of the gods who connected heaven and earth. Norse tradition called it Bifrost, the bridge between Asgard and the realm of mortals. In Hawaiian tradition, rainbows mark sacred places and ancestral presence. The Irish leprechaun pot of gold dates from medieval Celtic folklore, when buried treasure was a popular motif and the unreachable end of the rainbow made for a fitting hiding place.

The science came much later. Aristotle wrote about rainbows in the 4th century BCE. Alexander of Aphrodisias described the dark band between primary and secondary bows in the 2nd century CE. Persian mathematicians worked on the geometry in the 13th century. René Descartes published the modern explanation of the 42-degree angle in 1637. Isaac Newton finished the picture in 1672 by demonstrating that white light is composed of all the colors of the spectrum and a prism can separate them.

How to Photograph a Rainbow Well

Rainbows are notoriously hard to photograph cleanly because the bow is much fainter than your eye perceives it. A few tricks help.

• Use a polarising filter on your lens. The light reflected back from the droplets is partially polarised, so a polariser can either intensify or wipe out the rainbow depending on the rotation. Rotate the filter until the bow is at its brightest.
• Underexpose by half a stop. The contrast between the bow and the bright sky behind it usually fools the camera’s auto-exposure into making the rainbow too pale.
• Include a foreground. A tree, a barn, a lighthouse anchored at the base of the bow gives the photograph scale.
• Move quickly. Rainbows usually last 5 to 15 minutes before the rain or the sun moves out of the right geometry.

When You Are Most Likely to See One

Rainbows are most common in spring and summer in the eastern half of the country, where afternoon thunderstorms drop rain ahead of westerly clearing skies. The Pacific Northwest produces rainbows almost daily in the wetter months because the same coastal storm cycle alternates rain and sun. The Plains see frequent rainbows around evening thunderstorm cells. Hawaii is famous for them; the islands’ volcanic peaks force trade-wind moisture into rain showers under nearly perpetual sun.

The Farmers’ Almanac long-range forecast covers seasonal precipitation patterns months ahead. The forecast does not predict the specific moment a rainbow will form, but it does flag the weeks where afternoon rain-and-sun cycles are most likely.

Watch the sky after the rain. The bow is usually there if you turn your back to the sun.

Frequently Asked Questions

Jaime McLeod

Jaime McLeod is a longtime journalist who has written for a wide variety of newspapers, magazines, and websites, including MTV.com. She enjoys the outdoors, growing and eating organic food, and is interested in all aspects of natural wellness.

• Jaime McLeod

  

  June 27, 2025

  10 Signs A Deceased Loved One Is Near
• Jaime McLeod

  

  August 22, 2016

  Make Your Own Pickles – It’s Easy!
• Jaime McLeod

  

  February 29, 2016

  What Is Winter Inversion?
• Jaime McLeod

  

  November 22, 2015

  Not All Sunrises are Created Equal