Earth and Space

Why is the Sky Blue?

Practical Activity for 11-14 PRACTICAL PHYISCS

This demonstration is an easy way to show how gas molecules in the atmosphere scatter light to cause the sky to appear blue.

Apparatus and Materials

  • Ray box and power supply
  • Beaker of water
  • Milk and pipette
  • White card or screen

Health & Safety and Technical Notes

Read our standard health & safety guidance


  1. Set up the ray box so that it shines through a large beaker of water onto a screen. It helps in our demonstration to have a piece of card on top of the ray box so that light only escapes from it in the direction of the beaker.
  2. You will need to adjust the distance between the ray box, beaker and screen to get the best focus of light on the screen. The beaker acts like a lens, and a good result is usually obtained if you produce a thin focused line of light on the screen.
  3. It only needs a few drops of milk to begin to see the effect of the bluer light being scattered and a more orangey light colour apprearing on the screen. You can add more milk if it helps, and make sure the milk is stirred in to the water.

Teaching Notes

  • Explain that white light is made up of a number of colours (more specifically red, green and blue) then it is easy to see that if the Earthʼs atmosphere scatters some blue light, then we are left with more red and green. If you mix red and green light you get yellow/orange light and this is what we see on the screen.
  • You can also see the yellow/orange nature of the light if you look through the beaker towards the ray box.

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