Rainbows form when sunlight interacts with countless tiny water droplets in the air, bending and splitting the light into different colors that return to your eyes at just the right angles.

Quick Scoop: Why rainbows form

Think of a rainbow as sunlight taking a detour through raindrops.

The basic recipe

  • You need three things: sunlight, water droplets in the air (like after rain, mist, or spray), and an observer with the sun behind them.
  • Sunlight looks white but actually contains many colors (red, orange, yellow, green, blue, indigo, violet).
  • When this light enters a spherical water droplet, three key things happen:
    1. It is refracted (bent) as it goes from air into water.
    2. It is reflected off the back of the droplet like a tiny curved mirror.
    3. It is refracted again as it exits the droplet back into the air.

Because each color bends by a slightly different amount, the white sunlight spreads out into a spectrum of colors, which we see as a rainbow.

Why it’s a big arc, not tiny dots

  • Every droplet sends different colors in different directions, but you only see one color from each droplet, depending on the angle between you, the drop, and the sun.
  • Droplets at about 42 degrees from the line opposite the sun send red light to your eyes; slightly lower angles send the other colors down to violet on the inner edge.
  • Because there are millions of droplets in a huge 3D region of sky at just the right angles, you see a continuous, curved band instead of lots of tiny separate rainbows.
  • Geometrically, the rainbow is actually a circle; from the ground we usually see only the upper arc because the rest is below the horizon.

Why the sun must be behind you and low

  • You see a rainbow when you look at raindrops in the part of the sky opposite the sun, so the sun must be roughly behind you.
  • The rainbow angle (around 42 degrees for red) means that if the sun is too high in the sky, the whole circle is pushed below the horizon and no visible rainbow appears.
  • This is why rainbows are most common in early morning or late afternoon, when the sun is lower.

Double and other special rainbows

  • Sometimes light reflects twice inside the droplets instead of once, creating a secondary, fainter rainbow above the main one.
  • In that secondary rainbow, the color order is reversed: red is on the inner edge and violet on the outer edge.
  • Under more unusual conditions (like different droplet sizes), you can even get twinned or supernumerary rainbows with extra fringes or separate arcs.

A simple way to picture it

Imagine each raindrop as a tiny prism-mirror combo that catches white sunlight, splits it into colors, and throws one particular color toward your eye at a very specific angle.

All the drops that happen to be at the “red” angle form the red band you see, all the ones at the “blue” angle form the blue band, and together they create the familiar multicolored bow across the sky.

Bottom note: Information gathered from public forums or data available on the internet and portrayed here.