Why is the sky blue?


  • Transparent box
  • Water
  • Flashlight
  • A few drops of milk (or powdered milk)
  • A partner


  1. Fill the box with water
  2. Shine the light through the water
  3. Add a few drops of milk (or pinches of powdered milk) one at a time into the water until you can clearly see a beam of light.
  4. Look at the beam from the side of the box and have your partner look at the beam from the end of the tank.
  5. Tell each other what color the water appears to be.


If you were looking at the beam from the side, the water should have looked blue. Meanwhile, your partner should have seen yellow water. This is because of how the light is being scattered. Like the sun, your mini flashlight emits white light which is made up of every color in the rainbow. Because blue has a shorter wavelength than red, more of it is scattered. The reason why your partner saw yellow light, though, is because it took longer for the light to reach them than it took to reach you. By the time it got to them, the blue light was too scattered and they were able to see colors with longer wavelengths.