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Microscope: Rainbow weight

By , 5 September 2013

Welcome to Double Helix magazine’s Q&A section – Microscope. We take a close look at small questions full of big ideas.

Q: What is the weight of a rainbow?
The Helix reader, Russyl from Tasmania

Stylised picture of a rainbow on a set of scales

Rainbows are pretty light!

A rainbow is caused by light bouncing inside raindrops. When sunlight hits a raindrop, some of the light is reflected. Different colours are reflected at slightly different angles; so, each colour remains separated over the whole rainbow. To see a rainbow, there needs to be water droplets in the air and white light coming from the same direction – usually from the Sun.

Every person looking at a rainbow sees one that is slightly different, because they are standing in a slightly different spot. If you and a friend walk in different directions while looking at a rainbow, you will each see the rainbow move differently. So, a rainbow is not a ‘real thing’ – it’s an illusion of light. And, light doesn’t weigh anything!

How much water does it take to make a rainbow? On a sunny day, grab a spray bottle filled with water. Go outside, and turn your back to the Sun. Put your spray bottle on the mist setting and squirt it all over the place – hopefully you will see a rainbow using only a few grams of water. However, a rain shower that drops one millimetre of rain over an area of one square kilometre contains 1000 tons of water. Big storms would carry many times more water.

If you have a nagging little query that needs an answer, send it in!

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