Rainbow

A rainbow is an optical phenomenon that appears in the sky when sunlight shines on water droplets in the air. This can happen during or after rain, as well as in mist, spray, or dew. The light bends and reflects inside the droplets, separating into the colors we see as a beautiful arc of colors.

Rainbows usually look like colorful arcs, but they can actually form full circles. Most of the time, we only see part of the circle — an arc above the ground. The colors in a rainbow go from red on the outside to violet on the inside. Sometimes, if conditions are just right, we can see a second, fainter rainbow outside the first one, with the colors in reverse order.

The longest-lasting rainbow ever recorded lasted almost nine hours. It was seen in the Yangmingshan range of Taipei, Taiwan in November 2017, according to the Guinness World Records. Rainbows are not just beautiful; they also show us how light and water work together in nature.

Visibility

Rainbows appear when there are water drops in the air and sunlight shines from behind you at a low angle. This is why you usually see rainbows in the western sky in the morning and in the eastern sky in the early evening. The best rainbows happen when part of the sky is still dark with clouds and you are in a spot where the sky is clear toward the sun. This makes the rainbow stand out brightly against the dark background. Sometimes, you can even see a fainter, larger rainbow outside the main one.

You can also see rainbows near waterfalls or fountains. Rarely, you might see a moonbow on a very bright moonlit night, which often looks white because our eyes don’t see color well in low light. From an airplane, you might see a rainbow as a full circle. The sky inside a rainbow is brighter than the sky outside because of how light scatters from raindrops.

Main article: Rainbow Further information: Rainbow hunt, Rainbow symbolism, Supernumerary rainbow, Double rainbow

Number of colours in a spectrum or a rainbow

See also: Spectral color

People often think of a rainbow as having seven colours: red, orange, yellow, green, blue, indigo, and violet. This idea comes from the scientist Isaac Newton, who used a mnemonic, Richard Of York Gave Battle In Vain, to remember them. Sometimes people use the reverse order, VIBGYOR.

But the number of colours isn't fixed. Newton first described five main colours and later added two more to make seven. The way we see colours in a rainbow can also depend on our language, meaning different people might notice different numbers of colours.

Explanation

When sunlight hits a raindrop, it enters, reflects off the back, and then leaves the drop. As it leaves, the light spreads out over a wide angle. The brightest part of this spread happens around 42 degrees from the direction opposite the sun. This is why we see rainbows as arcs in the sky.

White sunlight is made of many colors, and each color bends a different amount as it passes through the raindrop. Red light bends the least, so it appears on the outside edge of the rainbow arc. Blue light bends more, so it appears on the inside edge. This separation of colors is what gives the rainbow its beautiful multi-colored appearance.

Variations

A double rainbow happens when we see two rainbow arcs in the sky. The first rainbow is bright and close to us, while the second is fainter and appears higher up. The second rainbow shows its colors in the opposite order compared to the first.

Sometimes, very rare twinned rainbows can appear. These look like two rainbows side by side, starting from the same spot. Their colors stay in the same order as a normal rainbow.

From high places like tall buildings or airplanes, we can sometimes see a full-circle rainbow. Usually, we only see half of a rainbow, but from above, more of the circle becomes visible.

When the sun is low, we might see supernumerary rainbows. These are thin, extra colored bands that appear close to the main rainbow, often in soft colors like pink and purple.

A reflected rainbow can appear in calm water below a normal rainbow, mirroring its colors. A reflection rainbow is even rarer and appears above the horizon when sunlight reflects off water before reaching the rain droplets.

Very rarely, we might see a monochrome rainbow, which looks red because blue and green colors have been scattered away.

Higher-order rainbows, like third or fourth-order rainbows, are very faint and hard to see because they appear close to the sun. They have been photographed, but are usually too dim to spot with the naked eye.

Rainbows can also appear when the moon shines, called moonbows. These are much fainter and often look white because our eyes can’t easily see their colors.

Fogbows look like large, pale rainbows and form when very small water droplets in fog scatter light. They often appear white with hints of color on the edges.

Sleetbows are rare rainbows that form when light passes through falling ice pellets instead of water droplets.

Brightly colored arcs called circumzenithal and circumhorizontal arcs are not true rainbows. They are created by light passing through ice crystals high in the atmosphere.

Some scientists think rainbows might also exist on other worlds, like Saturn’s moon Titan, where methane instead of water could create them.

Materials with different properties from water, like salt or plastic, can create rainbows with different shapes and sizes.

Scientific history

The ancient Greek scholar Aristotle was the first to study rainbows carefully. He tried to explain how they form, though his ideas had some mistakes.

Later, the Roman philosopher Seneca the Younger wrote about rainbows in his book Naturales Quaestiones. He noticed that rainbows always appear opposite the sun and can also be seen in water sprayed by oars or through small holes.

In the 13th century, Kamāl al-Dīn al-Fārisī and Theodoric of Freiberg gave better explanations. They showed that rainbows form when sunlight bends and reflects inside water droplets.

Later, Descartes and Isaac Newton added more discoveries, helping us understand rainbows better today.

Experiments

Experiments to study rainbows have been done for many years. One early scientist, Theodoric of Freiberg, used round flasks filled with water to mimic raindrops. Another scientist, Descartes, used a special flask called a Florence flask for similar experiments. Today, people still use a simple experiment called the Florence rainbow, where a water-filled flask is lit from one side to create a rainbow on a screen. These experiments help us understand how natural rainbows form, even though there are small differences compared to real rainbows in the sky. Similar experiments can also be done with cylindrical containers of water to explore how rainbows change under different lighting conditions.

Religion, culture and mythology

See also: Rainbow flag

Rainbows have appeared in many stories, myths, and cultures around the world. In the Epic of Gilgamesh and the Book of Genesis, a rainbow appears after a big flood. In Norse mythology, the rainbow bridge Bifröst links the world of people with the world of the gods. Different cultures have seen rainbows as special symbols. In some traditions, a leprechaun’s pot of gold is said to be at the end of a rainbow. The goddess Iris in Greek mythology is the personification of the rainbow, acting as a messenger between gods and people.

Rainbows are also used in flags and symbols. They have represented peace, unity, and pride in many places and times. Some groups have called themselves a "Rainbow Coalition" to show they work together even if they are different. Rainbows have even appeared in logos for companies and in artwork. In many cultures, pointing at a rainbow is thought to bring bad luck.