Why Rainbows Appear

Written by: Paige Leal

We always seem to recgonize a rainbow when looking towards the sky and seeing a flash of color, but few people realize the complexity and beauty of the rainbow. 

A rainbow is an optical phenomenon that appears as a colorful arc stretching across the sky. It happens when sunlight interacts with water droplets in the atmosphere. These droplets could come from rain, mist, or even spray from waterfalls. Importantly, a rainbow doesn’t exist in one fixed spot, it depends on where you are standing and where the sun is. 

How Rainbows Form: The Science

Rainbows start with refraction, the bending of sunlight when it enters a water droplet. Because water is denser than air, the light slows down and bends, separating into its different color wavelengths. Inside the droplet, the light reflects off the back surface. Then, when it exits the droplet, it refracts again, bending once more as it leaves the water. Because each color/wavelength bends by a slightly different amount, the light spreads out into a spectrum. That’s why we see red on the outside of a rainbow and violet on the inside.

Why Rainbows Are Curved Circles

Rainbows appear as curved arcs because of the geometry of light and droplets. The droplets that send light to your eye lie on the surface of an invisible cone, with your eye at the tip. The brightest part of a rainbow comes from light that exits the droplet at a specific angle. This angle is about 42 degrees for red light. From the ground, we usually see just a half-circle. But from high up, like in an airplane you might see a full circular rainbow. 

Double Rainbows & Other Types

Sometimes, you can see a double rainbow, this is a fainter, secondary bow above the main one. This happens when sunlight reflects twice inside the raindrop before it exits. In a double rainbow, the colors are reversed, red is on the inner edge, and violet is on the outer edge.
There are even more rare or special kinds of rainbows, like fogbows and circular rainbows, depending on the size of the droplets and the observer’s position. 

Why We See Rainbows

For a rainbow to appear, three things have to align:

1. Water droplets must be in front of you ( after it rains) 

2. The sun needs to be behind you.

3. The angle has to be right, because  if the sun is too high in the sky, the bow won’t form

Even though it feels magical, a rainbow is really just a trick of light and water. Different droplets reflect different colors back to your eyes, and since every droplet is at a slightly different position, every rainbow you see is unique to you. So next time you look up into the sky and see the magical rainbow, think about how every person sees this rainbow differently and no rainbows are the same.