The Westport Library Resource Guides: Rainbows: About

How Are Rainbows Formed?

Rainbows are a welcome sight after drab and dreary rainstorms. However, despite being one of weather's most beloved phenomena, they can't be forecast. These optical illusions form spontaneously when drops of water (raindrops, or mist from everything from lawn sprinklers to waterfalls) disperse light into its component colors via two processes known as refraction and reflection.

Ideal Conditions for Spotting a Rainbow

Creating a rainbow isn't as simple as mixing together sun and water; otherwise, rainbows would follow nearly all rainfall. It's how these two ingredients interact determines whether or not a rainbow forms.

Look Skyward Just After a Morning or Afternoon Rainstorm

One of the best times to find light and drops of water paired up is near the end of a rainstorm when the sun peeks out from behind rain clouds, and a sprinkle of raindrops are still floating in the air where they catch the sunlight.

As sunlight shines into a raindrop, light waves travel from air to water. Because water is denser than air, the light wave slows down and bends or "refracts" as it enters the raindrop. Once inside the bead of water, light travels to the curved back surface of the droplet, bounces or "reflects" off of it, then journeys back through the droplet and emerges out its other side. The light refracts again as it exits the droplet, and upon re-entering the air, scatters in all directions—up, down, and sideways—while journeying to observers' eyes.

It's this refraction that gives rise to the smorgasbord of colors rainbows are famous for. Remember that "white" light is composed of all the visible colors in the electromagnetic spectrum: red, orange, yellow, green, blue, indigo, and violet. Continue reading from Treehugger