How Are Rainbows Formed? The Science Behind the Colors

By: Austin Henderson  | 
Ever been awestruck by a rainbow stretching across the sky after a summer rain? You might not know that each color-lit arch is actually a brilliant demonstration of physics in action. Let's dive into the science behind rainbows — yes, including how are rainbows formed — and uncover the lore that surrounds these colorful arcs. Gary Yeowell / Getty Images

When you hear the word "rainbow," maybe you think about the clouds parting at the end of rainy day, or a Pink Floyd album cover, or a mythical pot of gold. But how are rainbows formed? The science is pretty straightforward.


What Happens Inside a Prism

It all starts with a prism. Imagine a triangular piece of glass or plastic. This prism is like a backstage magician in the rainbow-creating show. When white light enters one face of the prism, it's as if the magician whispers an incantation: "Separate!"

You see, light enters the prism, and the magic begins. This process of splitting is thanks to the refractive index of the glass. What's that, you ask? Every material, from air to water, has a different refractive index. This index dictates how much light bends as it travels from one medium to another.


Refraction and Dispersion: The Color-Making Trick

When light hits the prism, it bends, or refracts, thanks to the change in refractive index between air and glass. But it doesn't stop there; as the light continues through the prism, it separates into its various colors.

We call this process dispersion, and it's how a spectrum of colors — from red light with the longest wavelength to violet light with the shortest wavelength — is created.

What Are the Seven Colors of a Rainbow?

The colors of a rainbow are red, orange, yellow, green, blue, indigo and violet. It's the sequence you never forget: Roy G. Biv!


How Raindrops Become Tiny Prisms

How Are Rainbows Formed in Nature?

Good news: You don't need a prism to witness this spectral display. Mother Nature's got you covered with raindrops.

Yes, raindrops in the air act like tiny prisms.


Light passes through each water droplet, bending as it goes in and reflecting off the side before it exits. This sequence—entering, bending, and exiting—is a process called refraction.

The Angles and the Arcs: Understanding Optical Geometry

To visualize how rainbows formed in the sky, think about the angles. Light entering a rain droplet bends at specific angles, depending on its color. Red light exits at an angle of 42 degrees, while violet is slightly smaller at 40 degrees.

The different angles from multiple droplets form a complete circle of color in the sky — our beloved rainbow. Ever noticed that most of the time, you see only part of the circle? That's because the ground gets in the way!

The Double Rainbow

A double rainbow is what happens when light inside the droplets reflects twice, meaning you get a second reflection. The secondary rainbow appears outside the primary bow and has its colors reversed. If you're lucky enough to see one, it means the water droplets are just the right size for that second show.

What Does a Rainbow Symbolize?

In various cultures, rainbows are symbols of hope, usually representing the promise of upcoming rain. So the next time you see a rainbow, it's not just a scientific marvel — it's also a multicultural sign of good things to come!


How Our Eyes Perceive Rainbows

To be scientifically correct, a rainbow is not an object but an optical illusion. What reaches our eyes is visible light, which gets processed by the human eye into the vibrant bands of color we associate with a rainbow.


The Physics of Light

Light is composed of photons — tiny particles that behave both as light waves and light rays. When light refracts, or bends, it does so because its light waves are interacting with the medium they're traveling through — be it air, glass, or water.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.


The next time you spot a rainbow, you will see it in a whole new light. For more information, check out How Rainbows Work.


What Causes a Rainbow FAQ

What does a rainbow symbolize?
Rainbows mean different things in different cultures across the globe. In some cultures, rainbows are considered to be a symbol of hope. The hope comes in the form of upcoming rain.
What are the seven colors of a rainbow?
The sequence of the rainbow is made out of seven colors and never changes. The seven colors found inside the spectrum of a rainbow include: red, orange, yellow, green, blue, indigo and violet (ROYGBIV).
Why is it called a rainbow?
The contemporary word “rainbow” is said to originate from the old English word “renboga.” The word renboga is derived from the words “regn,” meaning rain, and “boga,” meaning arched.
How is a rainbow formed?
Rainbows are formed when light shines through water. The light is bent and reflected, often leading to the formation of the colors of the rainbow.
Who discovered the rainbow?
Credit goes to Isaac Newton. Through a series of experiments with prisms and sunlight, Newton discovered that white light is made up of many different wavelengths, or colors.
Can you ever see a full rainbow?
Typically, you'd see a full rainbow only if you had an unobstructed view of the sky and bright sunlight from a different angle. Sometimes from an airplane or a hilltop, people have witnessed the full circle!