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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.

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2 responses

  1. Gareth Brand Avatar
    Gareth Brand

    Given the rainbow is created by light passing through water droplets, the weight of the water droplets need to be taken into account, so this is a false answer

    1. David Shaw Avatar
      David Shaw

      Hi Gareth,
      Thanks for your comment! the third paragraph of the answer attempts to measure the mass of the water, but it really depends. It can be as little as a single squirt from a spray bottle, or as much as a whole sky of rain.

      for another estimate of the upper end of a rainbow’s mass, imagine a mist of a gram of water in a metre cube. Then scale that up to 2 kilometres high, 50 kilometres wide and 5 kilometres deep. 2,000 x 50,000 x 5,000 = 500,000,000,000 grams or 500,000 tons of water!

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