Rainbows happen when sunlight enters falling raindrops, bounces off the back of each drop, and exits at an angle that splits white light into its full spectrum of colors. Every raindrop acts like a tiny prism, bending different wavelengths by slightly different amounts, which is why you see distinct bands of red, orange, yellow, green, blue, and violet arcing across the sky.
How a Raindrop Splits Light
The process starts when a ray of sunlight hits the surface of a raindrop. Light travels slower through water than through air, and this change in speed causes the beam to bend as it crosses the boundary. This bending is called refraction, and it’s the same thing that makes a straw look crooked in a glass of water.
Once inside the drop, the light hits the far inner wall and bounces back toward the front, like a ball hitting the inside of a curved mirror. As the reflected light exits through the front surface of the drop, it refracts a second time. Each of these two refractions bends shorter wavelengths (violet, blue) more sharply than longer wavelengths (red, orange). The result is that white sunlight fans out into its full range of colors. This spreading of light by wavelength is called dispersion.
A rainbow isn’t really a continuous gradient of six or seven distinct colors. It’s a smooth, unbroken distribution of wavelengths, from red light at about 660 nanometers down to violet at roughly 410 nanometers. We label it “red, orange, yellow, green, blue, violet” for convenience, but what you’re actually seeing is every shade in between, blending seamlessly into one another.
Why Rainbows Always Form at 42 Degrees
Light exits raindrops at many different angles, but the greatest concentration of outgoing rays clusters between 40 and 42 degrees from the original direction of sunlight. Red light, which bends the least, exits at about 42 degrees. Violet light, which bends the most, exits at about 40 degrees. That’s why red always sits on the outer edge of the arc and violet on the inner edge.
To see these angles, you need the sun behind you and rain in front of you. The center of the rainbow’s circle is always at the “antisolar point,” the spot directly opposite the sun from your perspective (essentially, where your shadow’s head would be). The arc you see is the collection of all the raindrops in the sky that happen to be at that 40 to 42 degree angle relative to your eyes. Move ten feet to the left, and you’re actually seeing light from a completely different set of drops. Every person sees their own personal rainbow.
Why the Sun Has to Be Low
Since the rainbow forms a circle centered on the antisolar point, the height of that arc depends on where the sun sits. The lower the sun, the higher the antisolar point rises, and the more of the circle you can see. At sunset, you can see a full semicircle with the top of the arc 42 degrees above the horizon. As the sun climbs higher in the sky, the arc shrinks. If the sun is more than 42 degrees above the horizon, the entire rainbow drops below the horizon and becomes invisible from the ground.
This is why rainbows are most common in the late afternoon or early morning, when the sun is low. It’s also why you’ll almost never see one around midday in summer at lower latitudes.
Double Rainbows and the Dark Band Between Them
Sometimes you’ll spot a second, fainter arc outside the primary rainbow. This secondary rainbow forms when light bounces twice inside each raindrop instead of once before exiting. That extra reflection does two things: it weakens the light (so the second bow is dimmer) and it flips the order of colors. Red appears on the inner edge of the secondary bow, and violet on the outer edge, the reverse of the primary. The secondary bow sits about 10 degrees farther from the antisolar point and is roughly twice as wide.
Between the two arcs, the sky looks noticeably darker. This region is called Alexander’s dark band. Inside the primary bow, light scattered back from raindrops brightens the sky. Outside the secondary bow, the same thing happens. But in the zone between, no concentrated light is being directed toward your eyes, so it appears dim by comparison. The light that would fill that gap is being refracted at angles that create rainbows for observers at other positions.
Supernumerary Fringes
If conditions are just right, you might notice faint, pastel-colored bands hugging the inner edge of the primary rainbow, especially near the violet end. These are supernumerary arcs, and they can’t be explained by simple bending and bouncing of light. They’re caused by wave interference.
Pairs of light rays that enter a raindrop at slightly different points can exit at the same angle, traveling parallel paths toward your eyes. When these two waves arrive in sync, they reinforce each other and produce a bright fringe. When they arrive out of sync, they cancel and produce a dark fringe. The alternating pattern creates those delicate extra bands. Supernumerary arcs are most visible when the raindrops are small and uniform in size, because variation in drop size blurs the interference pattern.
Fogbows, Moonbows, and Dewbows
The same physics that creates a standard rainbow produces several less common variations. A fogbow forms in fine mist where the water droplets are smaller than about 0.05 millimeters. Drops that small can’t separate colors effectively, so instead of a vivid spectrum you get a broad, ghostly white arc. It has the same shape and geometry as a rainbow, just drained of color.
A moonbow works identically to a solar rainbow, except moonlight replaces sunlight. The colors are all technically there, but moonlight is so dim that your eyes rely on their low-light receptors, which don’t detect color well. The result is a pale, whitish arc that can be eerie to see in person.
Dewbows appear on grass after sunrise, formed by light refracting through individual dewdrops sitting on blades of grass. Because the drops are large enough to disperse light fully, dewbows display the complete rainbow spectrum. But since the drops are on a flat surface instead of scattered through the sky, the bow stretches out into a hyperbola shape rather than a circular arc.
The Conditions You Need
Three things have to line up for a rainbow to appear. The sun must be shining behind you. Rain, mist, or spray must be falling or floating in front of you. And the sun needs to be less than 42 degrees above the horizon. The drops themselves don’t need to be any particular size to form a basic rainbow, but larger drops (around 1 to 2 millimeters) produce the most vivid, saturated colors. As drops get smaller, colors fade and overlap until you’re left with the washed-out white of a fogbow.
You can create your own rainbow with a garden hose on a sunny afternoon. Stand with your back to the sun, spray a fine mist in front of you, and look for the arc at roughly 42 degrees from the shadow of your head. It’s the same optics, just a smaller canvas.