Rainbows And How They Work

Podcast Transcript

Few things in nature are as instantly recognizable as a rainbow.

For thousands of years, rainbows have inspired myths, religion, art, and science.

Yet behind those bands of color is an extraordinary interaction between sunlight, water, geometry, and the physics of light itself.

From double rainbows to full circular rainbows seen from aircraft, the science behind them is far more fascinating than most people realize.

Learn more about rainbows and how they work on this episode of Everything Everywhere Daily.

Rainbows are one of the most beautiful phenomena in nature. They can be seen after a rain shower, near a waterfall, or even if you are spraying water from a hose.

While everyone is familiar with what a rainbow is and what they look like, most people have no clue how they are formed or why they exist.

A rainbow is formed when sunlight enters many tiny water droplets in the air and is bent, separated into colors, reflected, and bent again before reaching your eye. The key processes are refraction, dispersion, and internal reflection.

Sunlight looks white, but it is actually made of many different wavelengths of light. Red light has a longer wavelength, violet light has a shorter wavelength, and the other visible colors fall between them.

When sunlight passes from air into water, it slows down because light travels more slowly in water than in air. This change in speed causes the light to bend. That bending is called refraction.

The amount of bending depends on the wavelength of the light. Violet bends slightly more than blue, blue more than green, green more than yellow, and red bends the least. This separation of white light into colors is called dispersion. It is the same basic effect seen when a prism splits sunlight into a spectrum.

Inside a raindrop, sunlight enters near the front surface of the drop. As it crosses from air into water, the light bends toward the normal, meaning toward an imaginary line perpendicular to the surface of the drop. The separated colors then travel through the droplet and strike the back inner surface. Some of the light passes out, but some reflects off the inside of the droplet and travels back toward the front.

When the reflected light exits the droplet, it returns to the air from the water. Since it speeds up as it leaves the water, it bends once again, this time away from the normal. This second refraction spreads the colors farther apart.

The important angle for a primary rainbow is about 42 degrees. More precisely, red light emerges from the raindrop at about 42 degrees from the direction opposite the Sun, while violet emerges at about 40 degrees. That is why red appears on the outer edge of the rainbow and violet on the inner edge. The other colors fall between those angles.

You see a rainbow only when the Sun is behind you and rain or mist is in front of you. The center of the rainbow lies on a line extending from the Sun through your head to the opposite point in the sky, called the antisolar point.

Every droplet that sends red light to your eye at roughly 42 degrees contributes to the red band. Droplets sending violet light at roughly 40 degrees contribute to the violet band. Millions of droplets are doing this at once, and your eye sees their combined light as a colored arc.

A rainbow is actually part of a circle, not truly an arch. The circular shape happens because all the droplets sending light to your eye at the correct angle form a cone around the antisolar point. From the ground, the lower part of the circle is usually blocked by the horizon. From an airplane or a mountain, it is sometimes possible to see a nearly full circular rainbow.

Double rainbows happen when light reflects twice inside the raindrop before exiting. The second reflection sends light out at a different angle, around 51-54 degrees from the antisolar direction. Because of the extra reflection, the secondary rainbow is dimmer and wider, with the color order reversed: red on the inside and violet on the outside.

While rainbows show the full spectrum of visible light, they are not the only natural phenomena that do so. There are other ways it can happen as well.

One of the most common is a halo around the Sun or Moon. These halos are created by ice crystals high in the atmosphere, usually in cirrus clouds. The crystals refract light in a way similar to water droplets, often creating a pale ring about 22 degrees around the Sun. Sometimes the edges show distinct reddish and bluish colors.

Sundogs, also known as parhelia, are bright colored spots that appear on either side of the Sun. They are also caused by ice crystals, but specifically by hexagonal crystals aligned horizontally in the atmosphere. Sundogs often display rainbow-like colors, especially red nearest the Sun and blue farther away.

Cloud iridescence occurs when sunlight passes through very small water droplets or ice crystals in thin clouds. The light diffracts around the particles, producing pastel bands of pink, green, blue, and yellow. The effect can resemble oil on water.

Glories are colorful rings sometimes seen around the shadow of an airplane on clouds or around a mountaintop shadow in fog. They are caused by backscattering and diffraction of light by tiny droplets. Unlike rainbows, glories form concentric circles centered on the observer’s shadow.

Not surprisingly, rainbows have been studied throughout history and have found places in the stories of various cultures.

In the biblical story in the Book of Genesis, the rainbow is presented as a sign of God’s covenant with Noah after the flood. It symbolized the promise that the Earth would never again be destroyed by a global flood.

One example of this is the Ancient Greeks, who had a goddess named Iris. Iris was the goddess of rainbows and messenger for the Olympian gods. Iris was viewed as a personification of rainbows, and it was believed that she was traveling on them when sending her messages. Her parents were a marine god and a cloud nymph, suggesting that the Greeks understood, to some extent, how rainbows functioned.

Across Norse and Japanese mythologies, the arched form of the rainbow led to its symbolization as a bridge. While these cultures interpreted the bridge differently—with the Japanese seeing it as a gateway to heaven and the Norse as a route between different realms—both utilized it to represent a physical link between the natural and supernatural worlds.

In Hindu and Buddhist Tantra traditions, rainbows are seen as a physical manifestation of those who have achieved the highest possible meditative state. When a person reaches this state, they attain a “rainbow body.” This is a state in which a person’s body dissolves into light at death. In this case, they experience total liberation and become pure essence. This is considered to be the final step before Nirvana, or the highest stage of enlightenment.

In many Indigenous Australian traditions, the Rainbow Serpent is a major spiritual figure associated with water, fertility, creation, and the shaping of the landscape. The Rainbow Serpent appears in numerous Dreamtime stories across different Aboriginal cultures.

In Chinese mythology, the goddess Nüwa was said to repair the broken sky using stones of five colors, and rainbows were sometimes viewed as evidence of this repair. Nüwa is one of the most important creator figures in Chinese tradition.

Irish folklore contains perhaps the most famous legends regarding rainbows, centered on the leprechaun. These beings originate from the Celtic religion, which was a pre-Christian polytheistic religion where divine entities were thought to derive their power directly from the natural world.

Often portrayed as elderly, tiny men who are expert craftsmen, leprechauns are said to belong to a race that lived in Ireland before humans arrived. They are frequently seen as personifications of nature and are famously known for possessing a concealed pot of gold.

This treasure is hidden at the rainbow’s end, guarded fiercely by these creatures. By choosing such a location, leprechauns demonstrate their wit; because a rainbow’s end can never actually be reached, the gold remains permanently inaccessible to humanity. According to legend, a leprechaun will only relinquish his wealth to someone capable of capturing him.

Symbolically, the rainbow in this story serves as a lesson. Because the rainbow is nothing but an illusion, the treasure underneath it is unattainable. It is a cautionary tale, essentially telling the audience that pursuing the treasure is impossible and that some dreams simply aren’t meant to come true.

The scientific study of rainbows also goes back thousands of years.

An early philosopher who studied rainbows was Aristotle. This is particularly evident in his work De Meteorologica. During his time studying rainbows, he speculated about how they formed, positing that the colors came from sunlight reflected in raindrops.

The next notable scientist to study rainbows was Theodoric of Freiberg in the 14th century. Theodoric was known for studying how rainbows formed by using a globe of water. He viewed the globe as a representation of a raindrop. When a ray of light entered Theodoric’s raindrop, it experienced a similar phenomenon to how waterdrops and light form rainbows in a natural setting.

While his research was fundamental in proving the refracting process, the first full explanation of how rainbows work didn’t occur until the 1630s. This was done by the French philosopher and scientist René Descartes.

Descartes studied rainbows using a boiling flask filled with water. He placed a screen with a hole in front of it. A white light beam shone through the hole, mimicking a sunbeam. When the beam hit the water, a rainbow appeared on the screen.

Descartes further proved his experimental findings with mathematical evidence. Descartes showed that the light that passed through the raindrop was emitted in different directions, approximately 42 degrees from its origin. Each color refracts differently, so some appear at a different ‘rainbow angle.’

Isaac Newton revolutionized the understanding of rainbows in the 17th century by showing that white light is composed of many colors. Using glass prisms, he demonstrated that sunlight could be separated into a spectrum and then recombined back into white light.

Newton realized that rainbows form because water droplets act like tiny prisms, refracting and separating sunlight into its component colors. His work helped replace earlier theories that held that colors were somehow created by water or the atmosphere itself.

During the 19th century, scientists developed the wave theory of light, which greatly improved the understanding of rainbows. Thomas Young showed that light behaves like a wave, helping explain interference and diffraction effects seen in phenomena such as rainbows.

Rainbows are among the few natural phenomena that have inspired mythology, religion, art, and science. What appears to be a simple band of color in the sky is actually the result of geometry, optics, atmospheric conditions, and the fundamental nature of light itself.

From ancient stories about divine bridges and serpents to Newton’s prisms and modern physics, rainbows have continually reminded us that even familiar things can contain extraordinary complexity. The next time you see one after a storm, you’ll know there is far more happening than just sunlight and rain.