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A rainbow is something we can see, most often when there is a mix of sun and rain showers. But how do rainbows form?

In this article you can learn:

• The different colours of light that we get from the sun
• What happens when light hits a raindrop
• Why rainbows are arch-shaped
• How you can create your own rainbow

This resource is suitable for weather topics for primary school learners.

Watch this video to discover more about rainbows, their shape and colours.

For rainbows to form, we need two things - sunlight and rain.

Sunlight

• Sunlight looks like white light but white light is actually made up of a spectrum of different colours mixed together.
• Rainbows appear when white light is spread out into all these different colours. This is called dispersion.

Rain

1. When sunlight enters a raindrop, the different colours are bent at different angles. We call this refraction.
2. The different colours reflect off the inside of the back of the raindrop.
3. When the sun is behind you and the rain is in front of you, a rainbow of colours is reflected back from the inside all the raindrops.
4. The seven main colours we see are red, orange, yellow, green, blue, indigo and violet.

We can separate the colours in white light easily by using a triangular glass prism.

1. Place the prism on a piece of white paper on a table
2. Shine a beam of light (for example from a torch) through the prism
3. Move the prism to change the angle that the light hits it at
4. When the angle is right, the prism disperses the light into a rainbow.

• Rainbows are curved because we see light reflected at different angles from raindrops all around the direction you are looking in.
• We usually see rainbows as arches. That is because the land gets in the way.
• If you're up in an airplane or at the top of a mountain, you might see a rainbow as a whole circle.

This slideshow shows some of the rainbows you might see after a rainy day.

• rainbow - an arch of different colours that happens when sunlight shines through rain.
• prism - a 3D object that is the same size and shape all the way through. A triangular glass prism can separate white light into different colours.
• refraction - the change of speed of light that can cause different colours to bend at different angles when it passes from air into glass or another material.
• dispersion - the separating of white light into a spectrum of different colours.
• spectrum - the range of colours that white light can be separated into.

Test your knowledge of rainbows with this quick quiz!

Create your own rainbow

You will need:

• A nice sunny day
• A water spray or garden hose

What to do:

Keeping the sun behind you, spray the water away from you. You will see a rainbow forming because of the water droplets in the spray.