• Question: Why is the sky blue?

    Asked by ruby to Duncan on 6 Nov 2017.
    • Photo: Duncan McNicholl

      Duncan McNicholl answered on 6 Nov 2017:


      Because air is blue. That’s why mountains that are far away look blue too; there’s lots of air in between you and them.

      If you’re interested in why air is blue, it’s to do with the size of the air molecules, and the size that they are means that they tend to scatter blue light and let red light carry on. That means that when you look at the sky, the light going across it from the sun has all the blue scattered away and towards you, and the red light carries on high up. It’s the same reason a sunrise or sunset is red: the air scatters the blue light away from you and leaves the red light to come straight at you.

      If you’ve got a big glass tank you can see this for yourself: fill it with water, and then put a spoonful or two of milk in it. When you shine a torch through it, it looks blue from the side and red f you look towards the torch.

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