Science Experiments

DIY Rainbow - how a rainbow forms

Sun, 28th Mar 2010

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What you Need

Plant Mister or spray hosepipe

A sunny day

What to do

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?

What may happen

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

 

 

Why does it happen?

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

Dave Ansell

Multimedia

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Comments

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Have you ever seen a circular rainbow? I saw one while riding in an airplane. I was looking toward the ground and noticed the shadow of the plane racing along the ground. A rainbow appeared in a perfect circle, with the plane's shadow in the center. That sight was worth the pot of gold I paid for the plane ticket! Mike, Mon, 26th Apr 2010

that sounds cool. Amazing nature johnttrick, Tue, 31st Aug 2010

That sounds about right, your shadow should always be at the centre of any rainbow, just normally there is not enough rain between you and the rainbow below you so you can't see it. daveshorts, Thu, 2nd Sep 2010

Full circle rainbow http://atoptics.wordpress.com/2011/06/07/full-circle-rainbow/ solarhalo, Sat, 25th Jun 2011

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