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podcast transcript
Few things in nature are as instantly recognizable as rainbows.
For thousands of years, rainbows have inspired mythology, religion, art, and science.
But behind these bands of color lies a special interaction between sunlight, water, geometry, and the physics of light itself.
From double rainbows to fully circular rainbows seen from airplanes, the science behind them is more fascinating than most people realize.
Learn more about rainbows and how they work in this episode of Everything Everywhere Daily.
Rainbows are one of the most beautiful phenomena in nature. They can also be seen after showers, near waterfalls, and when spraying water from a hose.
Everyone knows what a rainbow is and what it looks like, but most people have no idea how they form or why they exist.
A rainbow is formed when sunlight enters numerous tiny water droplets in the air, is refracted, separated into colors, reflected, and bent again before reaching the eye. The main processes are refraction, dispersion, and internal reflection.
Sunlight appears white, but it is actually made up of light of various wavelengths. Red light has longer wavelengths, purple light has shorter wavelengths, and other visible colors fall in between.
When sunlight passes from air to water, it slows down because light travels more slowly in water than in air. This change in speed causes the light to bend. This bending is called refraction.
The degree of bending depends on the wavelength of light. Purple bends slightly more than blue, blue bends more than green, green bends more than yellow, and red bends the least. Separating white light into colors is called dispersion. This is the same basic effect that occurs when a prism splits sunlight into a spectrum.
Inside a raindrop, sunlight enters near the front of the raindrop. When light passes from air to water, it bends in the normal direction. That is, it points toward an imaginary line perpendicular to the surface of the drop. The separated colors travel through the water droplets and reach the inner surface of the back. Some of the light disappears, but some is reflected inside the droplet and travels back to the front.
When the reflected light exits the water droplet, it returns from the water to the air. As it leaves the water it picks up speed so it bends once again, this time off the top. This second refraction spreads the colors further apart.
The critical angle of a primary rainbow is approximately 42 degrees. To be more precise, the red light in a raindrop comes out at an angle of about 42 degrees away from the sun, and the purple light comes out at an angle of about 40 degrees. This is why red appears at the outer edge of the rainbow and purple appears at the inner edge. Other colors fall between those angles.
You can only see a rainbow when the sun is behind you and rain or fog is ahead. The center of a rainbow is on a line extending from the sun overhead to a point opposite in the sky, called the antisolar point.
All the water droplets send red light into the eye at an angle of approximately 42 degrees, forming a red band. Water droplets emitting purple light at an angle of approximately 40 degrees form a purple band. Millions of water droplets do this simultaneously, and the eye sees the combined light as an arc of color.
A rainbow is actually part of a circle and not a true arch. The circular shape occurs because all the water droplets that send light into the eye at the right angle form a cone around the sun’s opposition point. On the ground, the lower part of the circle is usually blocked by the horizon. Sometimes you can see an almost completely circular rainbow from an airplane or a mountain.
A double rainbow occurs when light reflects twice inside a raindrop before exiting. The second reflection sends the light at a different angle, about 51 to 54 degrees away from the sun. Because of the additional reflections, the second rainbow is darker and wider, and the color order is reversed. It is red on the inside and purple on the outside.
Rainbows display the entire spectrum of visible light, but they are not the only natural phenomenon to do so. This can happen in other ways too.
One of the most common is the halo around the sun or moon. These halos are typically created by ice crystals in the upper atmosphere of cirrus clouds. The crystals refract light in a similar way to water droplets, often creating a pale ring around the Sun about 22 degrees deep. Sometimes they have distinct red and blue edges.
Sundogs, also known as parhelia, are brightly colored spots that appear on either side of the sun. This is caused by ice crystals, but especially by hexagonal crystals arranged horizontally in the atmosphere. Sundogs often display rainbow-like colors, especially red closest to the sun and blue farther away.
Cloud iridescence occurs when sunlight passes through tiny water droplets or ice crystals in thin clouds. Light diffracts around the particles, creating pastel bands of pink, green, blue and yellow. The effect can be similar to oil on water.
Glory is a colorful ring that can sometimes be seen around the shadow of an airplane in the clouds or the shadow of a mountain top in the fog. This occurs due to backscattering and diffraction of light by small water droplets. Unlike rainbows, glories form concentric circles centered around the observer’s shadow.
Naturally, rainbows have been studied throughout history and have found a place in the stories of various cultures.
In the biblical story of Genesis, the rainbow is presented as a sign of the covenant God made with Noah after the flood. This symbolized the promise that the Earth would never again be destroyed by flood.
One example of this is the ancient Greeks, who had a goddess named Iris. Iris was the goddess of the rainbow and the messenger of the Olympic gods. Iris was considered the embodiment of the rainbow and was believed to be traveling on a rainbow when sending messages. Her parents were sea gods and cloud nymphs, which suggests that the Greeks had some understanding of how rainbows functioned.
In Norse and Japanese mythology, the arched form of the rainbow led to its symbolism as a bridge. Although these cultures interpreted the bridge differently (the Japanese saw it as a gateway to heaven, the Northern Europeans saw it as a path between different realms), both utilized the bridge to represent a physical connection between the natural and supernatural worlds.
In the tantric traditions of Hinduism and Buddhism, rainbows are considered the physical manifestation of people who have reached the highest possible state of meditation. When a person reaches this state, he or she acquires a “rainbow body.” This is the state in which a person dies and their body melts into light. In this case, they experience complete liberation and become pure essence. This is considered the final stage before nirvana, i.e. the highest stage of enlightenment.
In many Australian Aboriginal traditions, the Rainbow Serpent is a major spiritual figure associated with water, fertility, creation and landscape formation. The Rainbow Serpent appears in numerous Dreamtime stories across various indigenous cultures.
In Chinese mythology, the goddess Nüwa is said to have used five-colored stones to mend the broken sky, and rainbows were sometimes seen as evidence of this repair. Nüwa is one of the most important creators in Chinese tradition.
Irish folklore contains perhaps the most famous legend about rainbows, which centers around leprechauns. These beings originate from the Celtic religion, a pre-Christian polytheism in which divine beings were thought to draw their power directly from the natural world.
Leprechauns, often depicted as skilled artisans and little old men, are said to belong to a race that lived in Ireland before the arrival of humans. They are often seen as the personification of nature and are famous for possessing hidden pots of gold.
This treasure is hidden at the end of the rainbow and is fiercely guarded by these creatures. By choosing such a place, leprechauns show their wit. Since the end of the rainbow is virtually unreachable, gold remains permanently inaccessible to humanity. Legend has it that a leprechaun will only give up his wealth to anyone who can capture him.
Symbolically, the rainbow in this story teaches a lesson. Because the rainbow is just an illusion, the treasure beneath it cannot be obtained. This is essentially a story warning the audience that the pursuit of treasure is impossible and that some dreams cannot come true.
Scientific research on rainbows also goes back thousands of years.
The first philosopher to study rainbows was Aristotle. This is especially noticeable in his works. Meteorological Administration. While studying rainbows, he speculated about how rainbows form, postulating that their colors come from sunlight reflecting off raindrops.
The next notable scientist to study rainbows was Theodoric of Freiberg in the 14th century. Theodoric was famous for using water beads to study how rainbows form. He saw the Earth as a representation of raindrops. When a ray of light entered Theodoric’s raindrop, he experienced a phenomenon similar to the way water droplets and light form a rainbow in the natural environment.
Although his work was fundamental in proving the process of refraction, it was not until the 1630s that the first complete description of how rainbows worked was given. French philosopher and scientist René Descartes did this.
Descartes studied rainbows using a boiling flask filled with water. He placed a screen with holes in front of it. A white ray of light shone through the hole, imitating sunlight. When the beam hit the water, a rainbow appeared on the screen.
Descartes further substantiated his experimental findings with mathematical proofs. Descartes showed that light passing through a raindrop radiates in a direction about 42 degrees different from its origin. Each color is refracted differently, so some appear at different ‘rainbow angles’.
Isaac Newton revolutionized our understanding of rainbows in the 17th century by showing that white light is composed of many colors. Using a glass prism, he showed that sunlight could be split into spectra and then recombined back into white light.
Newton realized that rainbows form because water droplets act like tiny prisms, refracting sunlight and separating it into its component colors. His work helped replace early theories that color was produced by water or the atmosphere itself.
In the 19th century, scientists developed the wave theory of light, greatly improving our understanding of rainbows. Thomas Young showed that light behaves like a wave, helping to explain the interference and diffraction effects in phenomena such as rainbows.
Rainbows are one of 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 basic properties of light itself.
From ancient stories of sacred bridges and serpents to Newton’s prisms and modern physics, rainbows have been a constant reminder that even the familiar can contain tremendous complexity. Next time you see a storm pass by, you’ll realize there’s a lot more going on than sunshine and rain.
