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Blue Sky, Red Sunset

Colloids and the Scattering of Light

Answer the following questions based on the presentation and class discussion.

  1. What does it mean for light to scatter?
  2. What is a solution?
  3. Do solutions scatter light? Why or why not?
  4. What is a colloid?
  5. Do colloids scatter light? Why or why not?
  6. What is the most important thing that makes a solution different from a colloid?
  7. Light shines in straight lines through pure water or a solution. What happens to light passing through a colloid?
  8. Which color of light is scattered more: red or blue?
  9. How is the atmosphere of the Earth a colloid?

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  1. During the day the sky is blue and bright. Explain why using what you now know about the scattering of light and how it depends on the color of the light.
  2. At sunrise and sunset the sky near the sun is red or orange. Explain.
  3. Draw a diagram of the path light takes through the atmosphere at both noon and sunrise or sunset.
  4. How does the diagram improve your ability to answer the questions about why the sky is sometimes red and sometimes blue?
These questions accompany the Tyndall Effect demonstration as found in Chemical Demonstrations by Bassam Shakhashiri, vol. 3, number 9.41. Procedure B works well. A diagram of light scattering in the atmosphere can be found on Prof. Shakhashiri’s site at:
Last updated: May 26, 2010       Home