What makes a rainbow?


Rainbows are captivating. They are bright and colourful and look pretty in the sky but how are they made?

Well, to understand the answer, you need to know three things about light:

(1) Light from the Sun looks white (we call it ‘white light’) but it is actually a mixture of light of all the colours in the rainbow: red, orange, yellow, green, blue, indigo and violet.

(2) Light is made up of waves. Different colours of light have different wavelengths. Red has the longest wavelength and violet has the shortest.

(3) Light waves travel in straight lines. If something gets in their way, they can be reflected, bent or scattered and this can cause white light to become unmixed, allowing us to see its component colours.

Let’s get started by thinking about why the sky is blue. The atmosphere around the Earth is made up of a mixture of gases with tiny particles floating about in it. Light shining on the Earth from the Sun bumps into the gas molecules and the tiny particles and the light waves get scattered. Light waves at the blue end of the rainbow are scattered more than those at the red end because they have shorter wavelengths. Our eyes see these scattered blue light waves and that is why the sky looks blue to us.

So what makes a rainbow? Well, a rainbow can be seen in the sky when it is rainy and sunny at the same time. The raindrops get in the way of the sunlight and bend the lightwaves. As we know, waves of different lengths bend by different amounts so the white light becomes unmixed as it travels through a raindrop and splits into the different colours. The effect we see is an arc of colours in the sky – and we call it a rainbow!

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