DIY Rainbow - how a rainbow forms

28 March 2010

Ingredients

Plant Mister or spray hosepipe A sunny day

Instructions

On a sunny day find somewhere on the edge of the shade, and stand with your back to the sun.

Spray water into the sunny area and look through the spay into the shady area - othewise any effect will be washed out by the bright background. You may have to move your head around a bit.

What can you see?

Try moving your head, does the effect stay still relative to the world the water droplets or you?

Result

If you get it right you should find that you can see a rainbow, and as your head moves the rainbow moves with you.

DIY rainbow video

Explanation

You are making a rainbow in exactly the same way that nature does it, by shining sunlight onto water droplets. In a rainbow these water droplets are of course raindrops.

White light from the sun is made up of many different colours, and when the light hits an individual raindrop the light is refracted (bent) as it enters the drop because light travels more slowly in water than air, some of it is reflected from the back of the drop and then is refracted again on the way out. 

Water refracts different colours different amounts so they end up leaving in different directions. You can see this effect if you look at the reflection of a light in a glass, just before the reflection disappears it changes colour at an angle of about 40° to the incoming light.

Light and a water drop
Light hitting a drop of water can get reflected and refracted in different directions depending on its colour You can see a similar effect as the reflection of the light in the glass chages colour just before it disappears at the end of this video.

 A raindrop is of course a sphere so the light is actually reflected out in a way that is symmetrical around the incoming sunlight - it is reflected in a series of cones. This means that if you look at the raindrop from different places it looks different colours.

Cones of Light Many cones of light
Because the droplet is spherical, the refraction creates cones of light heading in different directions. So it will appear different colours if you look at it from different directions. So if you look at several drops at different angles they will appear different colours

 So if you view many water droplets you will see different ones from different directions, so they appear different colours. If you see millions of droplets, in some directions all of them will appear coloured - you will see a rainbow.

A rainbow in droplets
If you look at thousands of droplets they appear different colours in different directions and form a rainbow

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