• Question: What makes a rainbow?

    Asked by lucy phillips to Stewart, Miriam, Marton, Laura, Kathryn, David on 13 Nov 2018.
    • Photo: David Ho

      David Ho answered on 13 Nov 2018:


      We see rainbows because when light from the sun, which contains all the colours mixed up, passes through water, different colours travel at different speeds. This causes them to separate out and gives the familiar rainbow pattern!

      The physics term for the fact that different colours (wavelengths) travel at different speeds through materials is called “refraction”

    • Photo: Stewart Martin-Haugh

      Stewart Martin-Haugh answered on 13 Nov 2018:


      When light rays hit water droplets in the air, they get scattered away and change wavelength as they do so. The details are a little bit complicated (and David explains them well!), but what’s nice is that we now know that there’s a lot of different physics: rainbows, magnets, electricity, light, lasers that can all be explained by a few equations and a lot of calculation. It took until the 1950s for a theory to explain all of these things together – the theory is called Quantum Electrodynamics.

      Many physicists are trying to find a theory that can explain everything: gravity, dark energy/dark matter in the universe, etc – this is in some ways the ultimate goal of physics. People like me run experiments to try to test the predictions of some of these theories.

    • Photo: Miriam Hogg

      Miriam Hogg answered on 15 Nov 2018:


      We get rainbows when it is raining and sunny at the same time. This is because the falling rain and raindrops in the clouds scatter the light from the sun and creates a rainbow. The rainbow will always be in the opposite direction of the Sun, so if you want to find a rainbow stand with your back to the sun and you will normally find it!

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