How does a rainbow form?

Sunlight is a mix of colours

Sunlight looks white, but it is really a mixture of all the colours you see in a rainbow. Light travels in waves, and each colour has its own length of wave, called a wavelength. Red light has the longest wavelength that we can see. Violet has the shortest. When white light is split up into its colours, the band of colours is called a spectrum.

A rainbow is what you get when raindrops split sunlight into a spectrum, with millions of drops doing it at once. People often say a rainbow has seven colours: red, orange, yellow, green, blue, indigo and violet. Some people remember them with the name ROY G BIV. In real life the colours blend smoothly into each other, and there are no sharp lines between them.

A trip through a raindrop

Follow one ray of sunlight into a raindrop. First, as the light goes from air into water, it slows down a little, and that makes it bend. This bending is called refraction. Next, the light hits the back of the drop and bounces off, a bit like a mirror. This bouncing is called reflection. Finally, the light bends again as it leaves the drop and heads back out, towards your eyes.

Here is the clever part. Each colour bends by a different amount. Violet bends the most, and red bends the least. So the colours leave the drop spread apart, each one heading off in a slightly different direction. A raindrop is round, not a tiny prism with flat sides, but it splits light for the same reason a prism does: different colours bend by different amounts.

You never see all the colours from one drop. Think about the spot in the sky that is exactly opposite the Sun. Raindrops about 42 degrees away from that spot send red light to your eyes. Drops a little lower, about 41 degrees away, send green. Drops about 40 degrees away send violet. Millions of drops, each sending you one colour from its own place in the sky, build up the bands of the rainbow.

Why a bow, and why your back to the Sun?

All the drops that are 42 degrees from the spot opposite the Sun lie in a circle, so a rainbow is really a circle of colour. The ground gets in the way, so from down here we usually see just the top part, which looks like a bow. From an aeroplane, in the right conditions, you can see the whole circle.

To see a rainbow you need three things. The Sun must be behind you. There must be rain, or other tiny drops of water, in front of you. And the Sun must be less than 42 degrees above the horizon, or the bow would sit below the ground. That is why rainbows show up on partly cloudy days, when the rain cloud has gaps for sunshine to squeeze through. It is also why they are seen most often in the morning or late in the afternoon, when the Sun is low.

Your rainbow is yours alone. The angle of the light depends on where you stand, so two people side by side see slightly different rainbows. If you move, your rainbow moves with you. That is why nobody can ever reach the end of a rainbow. A rainbow is not an object that sits in one place, so there is no pot of gold to find. It is light arriving at your eyes.

Double rainbows and other tricks

Sometimes you see two rainbows at once. In some drops the light bounces off the inside twice before it escapes. The second bounce sends the light out at a different angle, about 50 degrees, so the second bow appears outside the first one. It is fainter, because some light is lost at each bounce. Its colours are also in the reverse order, with red on the inside edge and violet on the outside. Look at the photo of Sydney to see a double rainbow for yourself.

You do not need rain to make a rainbow. Any cloud of tiny water drops will do. The mist around a waterfall, or the spray from a garden sprinkler or hose, can make a small one. Fog can make a fogbow. Fog drops are much smaller than raindrops, so the colours blur together and a fogbow often looks white. And if the Moon is bright and low in the sky, its light can make a rare, faint rainbow at night. It is called a moonbow.

Rainbows and stories in Australia

Many First Nations peoples across Australia share stories of a Rainbow Serpent, a powerful creation ancestor who is linked to water. In Kakadu National Park, in the Northern Territory, the Rainbow Serpent made waterholes and rock passages as she travelled across the land. Kundjeyhmi speakers call her Almudj, and Jawoyn speakers call her Bolung. In western and central Arnhem Land she is known as Ngalyod or Borlung, and she is mostly linked with billabongs, creeks, rivers and waterfalls. For the Barkandji people of the Darling River, she is the Ngatyi. The Barkandji artist Badger Bates made a steel sculpture of a Ngatyi blowing a rainbow. It was installed at the entrance of the Australian Museum in Sydney in 2023.

These stories belong to the peoples who hold them, and they are different in each place. They are about much more than rainbows. They teach about Country, about respect, and about looking after water. If you would like to learn more, the best way is to hear them from the communities themselves.

📚 Words to know

Refraction

The bending of light when it goes from one material into another, such as from air into water.

Reflection

Light bouncing off a surface. Inside a raindrop, the light bounces off the back of the drop.

Wavelength

The length of one wave of light. Red has the longest wavelength we can see, and violet has the shortest.

Spectrum

The band of colours you get when white light is split up, from red through to violet.

Fogbow

A pale, nearly white rainbow made by the tiny water drops in fog or mist.

🧪 Try this: make a rainbow in the garden

1. Pick a sunny day, in the morning or late in the afternoon, when the Sun is low. Wear a hat and sunscreen. Never look straight at the Sun.
2. You need a garden hose with a spray nozzle, or a spray bottle, set to a fine mist. Spray over the garden or lawn, and use as little water as you can. Never spray people or pets.
3. Stand with the Sun behind you, so your shadow falls on the ground in front of you. Spray the mist ahead of you, the way your shadow points, and look into the mist. You should see a small bow of colour.
4. Take a step to the left, then a step to the right. What happens to your rainbow? Which colour is on the outside edge?
5. Challenge: turn around so the Sun is in front of you and spray again. Can you still see a rainbow? Use what you have learnt about where the Sun has to be to explain what you see.