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Why is the Sky Blue (or Red)?

Why is the Sky Blue (or Red)?

Credit: http://www.flickr.com/photos/26022173@N06/3490213937
Source: Adrian Tombu
License: CC BY-NC 3.0

Maybe you learned the answer to this question for a test. Refraction. But what does that mean? How can the sky be blue in one place and red in another?

Why It Matters

  • The electromagnetic spectrum is made up of many wavelengths of radiation. Different wavelengths have different energy: long wavelengths have lower energy than short wavelengths. We can only see a small part of the electromagnetic spectrum, the visible light. When visible light is mixed together it appears white.
  • Credit: wonker
    Source: http://www.flickr.com/photos/wonker/2505352564/
    License: CC BY-NC 3.0

    When white light enters a prism, the light is refracted into all the colors of the rainbow [Figure2]

     

  • When light hits the gas molecules in the atmosphere it scatters. But different wavelengths scatter different amounts. The amount of scatter depends on this factor, where \lambda is wavelength:

\frac{1}{\lambda^4} \qquad \text{The denominator is this:}  \qquad \lambda \ \times \lambda \ \times \lambda \ \times \lambda

  • This is the wavelength times itself four times. If the wavelength is large, that number will get really big. If the wavelength is small, that number will be much smaller. Since this factor is in the denominator though, small becomes big and big becomes small. For example, \frac{1}{2} = 0.5 \ \text{(one half)}, but \frac{1}{10} = 0.1 \ \text{(one-tenth)}
  • Since the wavelength of red light is longer than the wavelength of blue light, the scattering factor of red light is smaller for that of blue light. This is why the sky is blue! Blue light scatters more than red light! But why is the sky red at sunset?

Can You Apply It?

With the links below, learn more about scattering in the atmosphere. Then answer the following questions.

  1. Which wavelengths of sunlight are filtered out in the atmosphere?
  2. Why does blue light scatter more than other colors, except violet?
  3. Why doesn’t the sky appear violet?
  4. Why does the sky appear red, orange and yellow during a sunset?
  5. Why is a sunset more colorful if there is a nearby fire, if the air is polluted, or near a lake or ocean?

Image Attributions

  1. [1]^ Credit: http://www.flickr.com/photos/26022173@N06/3490213937; Source: Adrian Tombu; License: CC BY-NC 3.0
  2. [2]^ Credit: wonker; Source: http://www.flickr.com/photos/wonker/2505352564/; License: CC BY-NC 3.0

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