After a heavy shower, the clouds break, the sun slips out and a curved band of colour appears across the sky. People stop their cars, point their phones and fall silent for a moment. So how does a rainbow form? It is not a solid object or a painted arch. It is sunlight being bent, reflected and split into colours by millions of tiny raindrops. This guide explains the science in simple steps, why it forms an arc, and how you can find one yourself.

What You Need to See a Rainbow

A rainbow needs three things at the same time:

• Sunlight: the sun must be shining, and it must be low enough in the sky (below about 42 degrees above the horizon).
• Water drops in the air: rain, drizzle, mist or the spray of a waterfall.
• The right position: the sun must be behind you and the rain in front of you.

This is why rainbows usually appear when a shower is moving away and the sun is shining from the opposite side.

How Does a Rainbow Form? 5 Simple Steps

1. Sunlight enters a raindrop. Sunlight looks white, but it is a mixture of all colours.
2. The light bends (refraction). Light slows down when it passes from air into water, so its path bends as it enters the drop.
3. The colours separate (dispersion). Violet light bends the most and red light bends the least, so the colours start to spread apart inside the drop.
4. The light reflects. It hits the back of the drop and bounces toward the front.
5. The light bends again and leaves. As it exits, it bends once more, which spreads the colours even further, and they travel toward your eyes.

Each raindrop sends all the colours out in different directions. You only see one colour from each drop. Red comes from drops that are higher in the sky, and violet comes from drops that are lower. Millions of drops together build the full band of colour.

Why Is a Rainbow Shaped Like an Arc?

The light leaves the drops at a fixed angle: red light comes out at about 42 degrees from the line that runs from the sun through your head to the shadow of your head on the ground. All the drops that sit at the same angle from that line form a circle. The ground usually cuts off the lower half, so you normally see only an arc.

From an aeroplane, a mountain top or a hill, when the ground does not block the view, you can sometimes see a rainbow as a full circle.

The Colours of the Rainbow

The seven-colour list is a tradition. In reality, a rainbow is a smooth, continuous blend, and the colours fade into each other without sharp borders. Many people find it hard to pick out indigo.

What Is a Double Rainbow?

Sometimes a second, fainter rainbow appears above the main one. Here, the light has reflected twice inside each drop before coming out. A double bounce makes the colours flip, so the secondary rainbow has violet on the outside and red on the inside. It appears at about 51 degrees. The sky between the two bows often looks darker than the rest, and this dark band is known as Alexander’s band.

Where and When Can You See a Rainbow?

• Early morning or late afternoon: the sun is low, so the rainbow rises high above the horizon. Around midday the sun is too high, and any rainbow would be below the horizon.
• Just after a shower: when the sun comes out while raindrops are still falling.
• Near waterfalls and fountains: the spray works like rain. A garden hose on a sunny day can make a small rainbow too.
• In the monsoon season: in Pakistan, the monsoon months from July to September often produce showers that clear quickly with the sun still out, which makes them a good time to look for one. The hills of the north also give wide views of the sky after a storm.

Rainbow Myths and Facts

Does a rainbow have an end?

No. A rainbow is not in one fixed place. It depends on where you stand. Two people next to each other actually see slightly different rainbows, because each person receives light from different drops. If you walk toward it, it moves away, so you can never reach the end of it, or any pot of gold.

Is it a physical object?

No. It is an optical effect. You cannot touch it, and you cannot photograph it from “the side”.

Can you see rainbows at night?

Yes, but they are rare. A moonbow is made by moonlight on rain. It is usually very faint and looks whitish to the eye, because our eyes see colour poorly in the dark.

Other Rainbow-Like Sights

• Fogbow: a pale, almost white arc seen in fog. The droplets are too small to separate the colours well.
• Sundog: a bright coloured spot beside the sun, made by ice crystals in high clouds.
• Halo: a ring around the sun or moon, also caused by ice crystals.

If you enjoy sky science, read about atmospheric pressure to see how pressure changes bring the showers that make rainbows possible.

Make Your Own Rainbow at Home

1. Choose a sunny day and stand with your back to the sun.
2. Use a garden hose with a spray nozzle, or a spray bottle, and make a fine mist in front of you.
3. Look at the mist. You should see a small rainbow, and it moves when you move.

Another way is to place a glass of water on a sunny windowsill, with a sheet of white paper on the floor. The water bends the light and may cast a small band of colour onto the paper.

Why Does the Sky Look Brighter Inside the Rainbow?

The area inside the primary bow is often brighter than the sky outside it. This is because the drops also scatter white light in that region, and the sum of all those light rays adds a soft glow. It is a small detail, but it shows how much is happening in a drop of water only a few millimetres wide.

Learn More

For further reading on how rainbows work, you can visit NOAA SciJinks. To check whether showers and sunshine are expected in your city, use the live weather and hourly forecast on WeatherWorld. Storms bring more dramatic skies too, so see our guide on what a tropical cyclone is.

Frequently Asked Questions

How does a rainbow form in simple words?

Sunlight enters raindrops, bends, bounces off the back and bends again as it leaves. This separates the white light into its colours.

Why is red on the outside of a rainbow?

Red light bends the least, so it leaves the drops at a larger angle. Violet bends the most, so it appears on the inside.

Why do we need the sun behind us?

The light comes back toward the same side it came from. The rainbow is always in the part of the sky opposite the sun.

Can a rainbow form without rain?

Yes. Any water drops in the air will work, such as mist, spray from a waterfall, a fountain or a garden sprinkler.

How long does a rainbow last?

Usually from a few minutes to about half an hour. It lasts as long as the sun and the falling drops stay in the right position.

Final Thoughts

A rainbow is a simple idea with beautiful results: sunlight, water and the right angle. Light bends as it enters each raindrop, bounces inside it and bends again on the way out, and every colour takes a slightly different path. The next time a shower ends while the sun is out, turn your back to the sun and look at the opposite sky. You may be rewarded with one of nature’s most graceful shows.