Why is the Sky Blue?

Have you ever wondered why the sky appears blue most of the time? Why does it not appear red or yellow during the day? In this experiment, we will use milky water to simulate the sky and observe what happens when light shines through the glass.

 MATERIALS:

  • Spoon

  • Water

  • Milk

  • Clear glass

  • Flashlight (phone)

TIME ESTIMATED:

10-15 minutes

DIRECTIONS: 

  1. Fill the glass about 3/4 full of water, and then add about half a teaspoon of milk to the water and stir well.

  2. Lift up the glass and place it upon the flashlight so that the light can shine through the bottom of the glass.

  3. Continue to shine the light through the bottom of the glass until the water appears to turn blue. 

  4. Dispose of the milky water and remove the flashlight.

THINK LIKE A SCIENTIST!

  1. Why is milk necessary for this experiment? Would the color of the light change if it is shined through a glass that is filled only with water? 

  2. Why does the light only change to blue and not other colors?

HOW DOES IT WORK? 

The milky water acts like the water vapor in the sky. When light from the sun hits particles of water vapor in the atmosphere, the light scatters off of the particles and hits Earth’s surface. While sunlight or the light from the flashlight looks white, it is actually made up of all the colors of the rainbow. Among these colors, blue light has shorter wavelengths and higher frequencies than the other colors. These characteristics lead the blue light to scatter more strongly than the others. For instance, blue light is scattered almost 10 times as efficiently as red light. When we look at the sky, we can only see the lightwaves that are scattered into other directions by the atmosphere. Since the frequency of scattered light is higher for blue light, the sky appears blue. 

In contrast, when we view the sunset, the opposite occurs. This is due to the fact that when the setting sun is at the horizon, it gets lower in the sky, and its light has to pass through more of the atmosphere to reach us, leading to an even higher frequency for the blue light to be scattered. The overly scattered blue light allows the reds and yellows to pass straight through to our eyes.

FURTHER EXPLORATION: 

This experiment lets you understand something you see everyday- the blue sky- using scientific ideas! This is something that scientists must do everyday, as they try to explain how and why the things they observed happened. The seemingly trivial inquiry of the underlying mechanism behind the daily phenomenon of a blue sky can actually be applied to many scientific and industrial applications. For instance, light scattering can be useful in air pollution analysis—in a polluted sky, small particles of sulfate, carbon, or mineral dust further amplify the scattering of blue light, creating a more spectacular sunset.

Looking for more fun at-home STEM activities for your young scientist? Check out our workbook full of exciting science experiments and empowering activities! 

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