• Question: Why is the sky blue?

    Asked by rachaell to Nazim, Adam, Catherine, Leila on 15 Mar 2012. This question was also asked by sophied.
    • Photo: Nazim Bharmal

      Nazim Bharmal answered on 15 Mar 2012:

      The sky is blue because light from the sun is made of lots of colours, but when the light “hits” (we call
      it scattering) the gas molecules in the atmosphere, like the oxygen and nitrogen, then more of the blue light gets scattered than red light.

      When you look away from the sun, you see more of this blue light so the sky looks blue (the “sky” looks black on the moon because there is no atmosphere). Now, clouds are made up of water drops that are much bigger than molecules, so the scattering is the same for all colours, and that is why they look white.

      Finally, and be REALLY CAREFUL here, but if did you hold up your thumb to the sun (so it is blocked) and look around its edge then you can see the aureole which is the scattering near the sun. This also looks white, and is again because the scattering isn’t so much affected by the colour of the light.

      That pretty much proves that to our eyes, the light from the sun is actually white. But the scattering makes the sky blue.

    • Photo: Adam Stevens

      Adam Stevens answered on 22 Mar 2012:

      Can’t add much to Nazim’s answer!

      Dust and water and stuff in the atmosphere scatter blue light more than red light. When the sun is high this makes the sky around the sun blue. When the sun is low, it has to go through more atmosphere, so the blue light gets scattered off in another direction.

    • Photo: Leila Battison

      Leila Battison answered on 22 Mar 2012:

      Light from the sun is white, which actually means it has all the colours contains within it, each with a different wavelength.

      When the light hits the atmosphere, the molecules in the air scatter the light in all directions, but it scatters different wavelengths (colours) more than others. More blue gets scattered than red, giving the sky a blue colour.

      On Mars, the sky is red because of the dust reflecting the red light – Mars hasn’t got enough atmosphere to have the same scattering effect.

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