How Rainbows Work

If you spray a fine mist with a garden hose on a sunny day, a tiny rainbow pops up right in front of you. The giant arc stretching across the sky works the exact same way. The real heroes are millions of floating raindrops left in the air after a storm. Small as they are, each droplet acts like a precision glass bead.

The magic starts the moment sunlight enters a drop. Moving from air into denser water slows the light down and bends its path, an effect called refraction. This bending splits the blended white sunlight, sending each color out at a slightly different angle.

Next, the light travels through the drop and hits the back wall. Some of it bounces off the inside surface like a mirror, which is the reflection step (the rest passes straight out). Finally, as the reflected light exits the front of the droplet back into the air, it refracts a second time. This final bend spreads the colors even wider before they reach your eyes.

Look closely at a standard primary rainbow, and you will always find red on the outer edge and violet on the inside. That order never changes because each color of light travels as a wave with a different wavelength.

Red light has a longer wavelength and bends the least when traveling through water. Violet light has a shorter wavelength and bends much more sharply. Separating light into distinct colors based on how much each wavelength bends is known as dispersion.

When light leaves a droplet, red exits at roughly a 42-degree angle relative to the incoming sunlight, while violet exits at about 40 degrees. Because red emerges at a slightly steeper downward slant, raindrops higher up in the sky are the ones that channel red light straight into your eyes. Droplets lower in the sky line up to send you violet light. That geometry makes the outer curve appear red and the inner rim violet.

Standing on flat ground, you only ever see a rainbow as a semicircular arch. That is because the horizon blocks the bottom half—the center of the circle sits below eye level, directly opposite the sun. When the sun hangs low, the arch climbs higher; when the sun rises high, the visible arc shrinks. If you are looking down from an airplane or a high cliff with the sun behind you and rain clouds below, you can sometimes spot a complete circular rainbow. A rainbow is really a cone of light beaming toward you at about a 42-degree angle around the line connecting your eyes to the sun.

On special days, you might also spot a fainter twin resting just above the main bow. This secondary rainbow happens when sunlight bounces twice inside each waterdrop before escaping.

Each internal bounce loses some light, which makes the secondary rainbow much dimmer. Bouncing twice also flips the ray path completely inside out. The secondary rainbow exits at roughly 51 degrees, sitting higher in the sky with its colors reversed: violet on the outside and red on the inside.