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The science of rainbows

It's wonderful to meet you! Before we start looking at how sunlight interacts with water droplets to paint those beautiful arcs in the sky, I'd love to learn a little bit about you so I can tailor our conversation perfectly.

Could you share a bit about your current background in science or physics? Are we looking for a friendly, high-level conceptual overview, or would you like to dive deep into the math and physics behind the light waves?

I prefer a simple, high-level conceptual overview

A friendly, high-level conceptual overview is the perfect way to appreciate this beautiful phenomenon! Let's start with the big picture: what a rainbow actually is and the exact recipe nature uses to create one in the sky.

Your path
The Physics of Light
Inside the Droplet: Refraction, Reflection, and Dispersion
Beyond the Primary Rainbow

The Perfect Recipe for a Rainbow

At its heart, a isn't a physical object you can touch or walk up to. Instead, it is a brilliant optical illusion, a cosmic projection that only appears when you have the perfect mix of two simple ingredients: sunlight and water droplets in the air.
To spot one, you have to be standing in a very specific sweet spot: the Sun must be behind you, and the rain or mist must be in front of you. The sunlight beams from behind you, enters the millions of tiny airborne water droplets ahead, and gets bounced back to your eyes, splitting into a glorious spectrum of colors along the way.

Continue to how water droplets act like tiny mirrors and prisms

To understand how water droplets build a rainbow, we have to look at them as tiny, multitasking marvels. Every single falling raindrop acts as both a prism and a mirror at the same exact time.

The optical path of a light ray refracting, dispersing, and reflecting inside a single circular water droplet.

When a beam of white sunlight strikes a water droplet, it doesn't just pass straight through. Instead, a three-step dance takes place inside the drop:
  1. The Entrance (Refraction): As the light transitions from the thin air into the denser water, it slows down and bends. Because white light is actually a mix of all colors, and each color bends at a slightly different angle, the light starts to fan out into its individual wavelengths—this is the water droplet acting as a .
  2. The Bounce (Reflection): The fanned-out light travels through the drop until it hits the back wall. Instead of passing through, most of this light bounces off the back inside surface of the droplet, just like a ball bouncing off a .
  3. The Exit (Dispersion): The bounced light heads back toward the front of the droplet and exits. As it steps back out into the air, it speeds up and bends one more time, spreading the colors out even wider so they are clearly separated by the time they reach your eyes.

Why do some colors bend more than others?

It all comes down to a fundamental rule of how light travels through different materials: different colors of light travel as waves of different lengths, and those wavelengths react differently when they hit a new medium like water.

Think of white light as a team of runners holding hands, all running at the same speed through the air. Each color in that white light is a runner with a different stride length, which we call a .

Red light has the longest wavelength (the longest stride), while violet light has the shortest wavelength (the shortest, quickest stride).

A horizontal diagram showing a beam of white light traveling through air and entering a denser medium at an angle, splitting into a red wave with a long wavelength and a violet wave with a short wavelength.

When this team of runners suddenly transitions from easy-to-run-on air into thick, heavy water, they all slow down. But because violet light has a much shorter, tighter wavelength, it interacts more with the water molecules and gets slowed down the most.

Since violet light slows down more than red light, it bends much more sharply as it enters and exits the water droplet. Red light, with its long strides, feels the drag of the water the least, so it bends the least. This physical difference in bending is called , and it is the exact reason why the colors spread out and always show up in the same order in a rainbow.

Why is red always on the outside of the rainbow?

It is a beautifully simple question, but the answer is actually a clever bit of geometry. Even though violet light bends the most inside each individual water droplet, red light is what ends up at the outer top edge of the rainbow because of the path the light has to take to reach your eyes.

The geometric pathways of sunlight refracting and reflecting inside two water droplets at different heights to form a rainbow.

As you can see in the diagram, every droplet of rain sends a full fan of colors (from red down to violet) bouncing back toward the ground. But because you are standing in one specific spot, your eyes can only catch one color from any single droplet. The droplets high up in the sky project their red light at a wider (42∘42^\circ) directly into your eyes, while their violet light (40∘40^\circ) passes harmlessly over your head. Conversely, droplets that are slightly lower down in the sky direct their violet light into your eyes, while their red light falls short. Because you see the red light coming from droplets higher in the sky and the violet light from droplets lower down, red naturally crowns the outside of the arc, with violet sitting on the inside.