Why is the Sky Blue?

Have you ever looked up at the sky and wondered why its blue during the day yet black at night? The explanation is simple and it also explains the beauty of sunsets and sunrise.


Flashlight, drinking glass, eye dropper, water, milk, spoon


  1. Fill glass with water
  2. In a darkened room, direct the flashlight through the side of the glass and through the water.
  3. Observe the color of the water
  4. Add 1 drop of milk to the water and stir.
  5. Again shine the light through the water.
  6. Observe the color of the water.


The light passes through the clear water, however the milky water has a light blue-ish tint.


The waves of color that make up white light are actually many different sizes (wavelengths). The particles of milk the the water are small enough to block and reflect the blue light waves. Those waves are bounced around and cause the predominant light that we see coming from the milky water. Nitrogen and oxygen in the Earth’s atmosphere, are also small enough to separate out the blue light waves from sunlight. The blue light scatters throughout the entire atmosphere making the sky look blue from the ground, and the Earth look blue from space.

The color in the milky glass isn’t perfectly blue because milk contains molecules of several different sizes, and larger molecules are also reflecting light. This same phenomena happens in the atmosphere when large amounts of dust and water vapor scatter more then just the blue light waves. Clean, dry air, free of dust and water vapor, scatters the most blue waves.

Posted By Astrid

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