How Rainbows Form

Sunlight is full of colours

Sunlight looks plain white, doesn't it? But here is an amazing secret: white light is not really one colour at all. It is many colours mixed together.

When all the colours are jumbled up together, our eyes see white. But if we can pull those colours apart and spread them out, we can see each one. That is exactly what a rainbow does — it splits sunlight into a beautiful band of colours.

Bending light: refraction

To understand rainbows, we need one big idea: light can bend.

Light travels in straight lines through the air. But when light passes from air into water, it slows down a little and bends. When it leaves the water and goes back into air, it bends again. This bending of light is called refraction.

You can see refraction yourself. Put a straw in a glass of water and look from the side — the straw looks bent or broken at the water line! The light from the straw is bending as it passes through the water.

Here is the clever part: different colours bend by different amounts. Red light bends the least. Violet light bends the most. The other colours bend by amounts in between. So when white light bends, the colours spread apart like a fan.

Inside a raindrop

When it rains, the sky is full of millions of tiny round raindrops. Each raindrop is like a tiny ball of water. When sunlight hits a raindrop, three things happen:

1. The light bends as it goes into the drop (refraction). The colours start to spread.
2. The light bounces off the inside back of the drop (reflection), like a mirror.
3. The light bends again as it comes back out, spreading the colours even more.

So each raindrop acts like a tiny prism, splitting white sunlight into all its colours and sending them back towards you. When millions of drops do this together, you see a giant arc of colour across the sky — a rainbow!

The seven colours, always in order

A rainbow always shows its colours in the same order, from the outside to the inside:

1. Red (top, outside)
2. Orange
3. Yellow
4. Green
5. Blue
6. Indigo
7. Violet (bottom, inside)

Red is always on the outside because it bends the least, and violet is on the inside because it bends the most. The order never changes. Some people remember it with the name Roy G. Biv — the first letters of each colour!

When can you see a rainbow?

You cannot see a rainbow just any time. You need two things at once:

• Rain (or water droplets) in front of you.
• The Sun behind you.

The sunlight comes from behind, enters the drops in front, and bounces back to your eyes. That is why you often see rainbows when the Sun comes out during or just after a shower. If you turn to face the Sun, the rainbow disappears!

Try it yourself: make your own rainbow

You can split sunlight at home with just water.

You will need: a shallow dish or glass of water, a small flat mirror, and a sunny day (or a torch).

1. Fill the dish with water and place it where sunlight shines in.
2. Put the mirror into the water, leaning at a slant.
3. Move the mirror until sunlight hits the underwater part of it.
4. Hold a piece of white paper or point it at a wall above the dish.
5. The water and mirror bend and split the light — a rainbow appears on the paper or wall! The water is acting like a prism.

Stay safe: never look directly at the Sun, and ask a grown-up to help handle the mirror near water.

What we learned

White sunlight is made of many colours. When light enters a raindrop it bends (refraction), and because each colour bends a different amount, the colours spread out. Millions of raindrops together make a rainbow with seven colours, always in the same order, seen when the Sun is behind you and rain is in front. Now you know the science behind the prettiest thing in the sky!

Want to explore more about light? See how light bounces in Mirrors and Reflection, and how light makes dark shapes in Light and Shadows.