Why is the Sky Blue?

Thinking & Engaging
Thinking & Engaging

Seeks to answer one of the most popular questions asked by kids and helps them to understand how the particles in the air affect how we live everyday even though they are invisible to our eyes. This activity addresses topics like wavelengths.


  • Clear drinking glass
  • Water
  • Bar of white soap (milk powder as substitute)
  • Flashlight that emits white light (ie. LED light)


(Try the Colours of the Rainbow activity for a way to frame this activity). Ask the participants: do you know why the sky is blue? Take a few answers and then ask the participants to name all the colours in the rainbow. Encourage them to remember it during the experiment.


  1. Fill the glass with water.
  2. Dissolve a bit of soap into the water until you reach a cloudy solution. If using milk powder, dissolve one teaspoon at a time until cloudy solution is reached.
  3. In a dark room, point the flashlight at the cloudy solution from the side.
  4. Observe the fluid from the side to see a hint of blue colour.


  • Find out why sunsets are red/orange: Fill a tall glass or casserole dish with the cloudy solution (or milk powder). While shining the flashlight on the dish from one side in a dark room, observe the blue colour close to the light change to yellow, then orange, the farther away from the light.


  • Discourage the participants from drinking the solution if made with soap.


There are many tiny particles floating in the water that our eyes cannot see, and the same goes for the dust in the air. Sunlight, and the white light from the flashlight, are made up of all colours of the rainbow (red, orange, yellow, green, blue, indigo, violet)  and when we shine the flashlight on our cloudy solution, the light scatters around from particle to particle. The distance the light has to travel from the particle to your eyes determines the colour you see. These are known as wavelengths. A shorter wavelength means that the colour is more scattered than the others and equals a blue colour, while a longer wavelength means the colour is less scattered than others and equals a more orange colour, (Enhanced: As the sun sets, it moves further away from you. The light has further to travel, which is why a sunset looks orange or red).

  • What other colours can you see in the sky? Why do we see those colours?