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What is a rainbow?

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.

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Video - Rainbows

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

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How does a rainbow form?

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 .

Rain

  1. When sunlight enters a raindrop, the different colours are bent at different angles. We call this .
  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.
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How to disperse light

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.
A glass prism on a white surface dispersing sunlight into a rainbow spectrum of colours
Image caption,
When white light is shone through a prism, it can be dispersed into a spectrum of different colours. (Menno van der Haven / Alamy Stock Photo)
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What shape is 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.
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Different kinds of rainbow

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

Image gallerySkip image gallerySlide 1 of 3, A rainbow going over houses, When there's sunshine and rain together, you might see a rainbow. The rainbow has an arch shape because the ground stops us seeing any more of its full curve.
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Key words

Key words about rainbows

  • 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.
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Test your knowledge

Test your knowledge of rainbows with this quick quiz!

Quiz

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Challenge

Challenge

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.

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