How rainbows work

When light, air and water come together in just the right way, sometimes a beautiful rainbow is created. A rainbow is a visible continuum of colors from red to violet in a long arc or “bow”.

Basic Concepts

To understand rainbows, it is important to know the concept of refraction and the role of a prism. In simplest terms, refraction is when light changes direction or appears to bend. Light refracts when if travels through various mediums; when it goes from one medium to another it may start traveling at a different speed – this makes it appear to bend. For example, as light passes through air, it travels at one speed, but when it gets to a crystal and shines through it, the direction of the light will change as it slows down in the crystal.

A prism is a glass or crystal that not only changes the direction of light entering it, but also separates white light into the individual colors making it up. This happens when the prism bends the light more than once so the different colors come out distinctly. Different color frequencies travel at different speeds in the prism, so once the white light disperses into its component colors, all of them are distinctly visible.

How a Rainbow Works

So, in a rainbow light is passing through a prism. In this case, the prism is not glass or crystal – it is a raindrop or droplet of water – but it works in a similar manner. When sunlight passes through a group of raindrops at certain angles, the individual colors of the rainbow become visible. Specifically, a rainbow is made of red, orange, yellow, green, blue, indigo and violet light waves.

Each person viewing a rainbow sees only one color being refracted by each raindrop. Every color is being separated from the white light, but some of the colors are coming out of the drop at angles where an individual cannot see them. For example, red light comes out of a raindrop at a 42 degree angle and violet light comes out at a 40 degree angle with the other colors at angles in between. This is how each person viewing the rainbow sees bands of color instead of seeing a speckling of all of the colors of the rainbow.

It takes more than water droplets and sunshine for a rainbow to occur; all of the elements must be properly positioned. The sun must be shining and should be positioned behind you as you look toward water drops in the air in front of you.

The Shape of a Rainbow

A rainbow gets its arched shape from the angle of the colored light being refracted out of each raindrop. So, the single color a person sees coming from a raindrop comes out at an angle between 40 and 42 degrees, accounting for the curved shape of the bow.

The air inside of the arc of a rainbow often looks brighter than the air outside the rainbow. This is due to the light being reflected around in the raindrops.

Double Rainbows

Sometimes a second rainbow is visible at the same time as a primary rainbow. This is generally called a secondary rainbow or a double rainbow. The colors in the secondary rainbow will appear in reverse order from those in the primary rainbow.