How Are Rainbows Formed?

Inspired by this beautiful rainbow I saw at a waterfall in Dominica, a Caribbean Island, I decided to write a Christmas post about rainbows.

The origin of rainbows is shrouded in legend in various cultures. The ancient Greeks believed that rainbows represent the goddess, Iris. In Chinese culture, a rainbow is an interface between earth and heaven. ¹ Are you familiar with the saying, searching for a pot of gold at the end of a rainbow? It comes from an Irish tradition¹ but became persistent in the common language. The pot of gold represents a goal you want to achieve in this sense. ²

Where does the colour of rainbows come from?

Nowadays there are scientific explanations for the formation of rainbows. According to the National Geographic are Rainbows formed when “sunlight strikes raindrops in front of a viewer at a precise angle (42 degrees)” (National Geographic, s.a.). Its appearance depends on your standing point and out of which direction the sun is shining. Usually, the sun or another source of light is “behind the person seeing the rainbow” (National Geographic, s.a.). ³

To understand the refraction of light needed to create a rainbow it is helpful to learn a little more about the electromagnetic spectrum. Check out my last post about the colour of sunsets and the sky if you are unfamiliar with the term.

In short, the electromagnetic spectrum consists of different wavelengths of sunlight. Our eyes perceive all rainbow colours combined as white light. Therefore, the light of the sun appears white. You already know that it needs water and a source of light to create a rainbow, this is because the water droplets refract the light. The light is then reflected by the droplets themselves. Because the wavelengths of the electromagnetic spectrum are reflected at non-identical angles, the spectrum separates, resulting in the rainbow. ³

Why are rainbows bent in a round shape?

The shape of the rainbow results also from the light reflection and refraction which change the direction of the waves. The refraction of the wave makes it appear curved. Light is refracted as it enters a water droplet. Afterwards, the back of the water droplet reflects the light. The reflected light is refracted again in multiple angles, creating a curved shape. Because water droplets are responsible for refracting light, the radius of a rainbow is determined by their size. ³

Fun fact 

Rainbows actually form a circle. From the ground, you can only see the light reflected by the water droplets above your individual horzion. Nobody sees the same rainbow, because the horizon of each person is different. On the contrary aircraft viewers can sometimes see the rainbows´ full circle. ³

Conclusion

To create a rainbow, you need a source of light, most often the sun, and water. Water droplets refract and reflect light, dividing the electromagnetic spectrum into its various colors. The curved shape of a rainbow is caused by the phenomenon of light being reflected at different angles by water.

References:

1. Rainbow Symphony, 2021. Legends of the Rainbow: Myths Throughout History. Available at:  https://www.rainbowsymphony.com/blogs/blog/legends-of-the-rainbow-myths-throughout-history [25.12.2022].
2. Learning English, 2021. Is There a ‘Pot of Gold at the End of the Rainbow’? Available at: https://learningenglish.voanews.com/a/is-there-a-pot-of-gold-at-the-end-of-the-rainbow-/5810673.html [25.12.2022].
3. National Geographic, s.a. Rainbow. Available at: https://education.nationalgeographic.org/resource/rainbow [25.12.2022].