Big O’ Glass of Sunset!

See why the sky is blue and a sunset is orange, all in a glass of milk!


  • Tall clear glass of water
  • ½ tablespoon of milk
  • Flashlight


1. Stir the milk into the water

pouring milk

2. Shine the flashlight from the side of the glass and observe the glass, what color is it? 


3. Look directly at the flashlight through the glass, what color is it?

sunset4. Shine the flashlight up through the bottom of the glass and look down from the top. What color do you see now?


  • If you imagine the flashlight at the sun and the glass as the atmosphere, when you looked at it from the side, what time of day did that correspond to?
  • What about when you looked at the flashlight end on or through the bottom of the glass?
  • Why does the sky look blue during the day but when you look at the sun during a sunset it looks orange?
  • Why do you think pollution, though very bad, makes more beautiful sunsets?


When light interacts with a particle, some of it is scattered while some goes through. Lord John W. S. Rayleigh (1842-1919) discovered that some colors of light are scattered more than others. In our atmosphere, blue light is scattered more than red or yellow light. During the day, we don’t look directly at the sun, so we are only seeing the scattered light, which is blue. However, during a sunset, when the sun is low in the sky, we are looking at it more directly. We see the yellow and orange light that is not being scattered but passing through the atmosphere to our eyes.

In this experiment, the milk particles are doing the scattering. Just like the light from the sun, the light from the flashlight is made up of many colors, just like the light from the sun. The milk in the glass is scattering the blue light but allowing the yellow and orange light to pass through. This makes the milk appear blue when looked at from the side, but yellow when looking straight at the flashlight.

Suggested Resources:


WhySkyBlue Enterprises, Why is the Sky Blue?

Ask A Physicist: Explain how beautiful sunrises and sunsets are the result of dust in the atmosphere?


Cobb, Vicki, and Cobb, Josh. Light Action!. New York: HarperCollins, 1993