Harvard University Astronomy Lab and Clay Telescope

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Why is the sky blue? Why are sunsets (and dusty stars) red?

SPU-21 Fall 2014

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This exercise will help you explore something about the colors of the Sun, by a simple analogy. The yellowish Sun of course has all colors, from violet to red, and beyond to both much shorter wavelengths (ultraviolet, x-rays, gamma rays, ...) and much longer wavelengths (infared, millimeter wave, radio, ...). The Earth's atmosphere, by preferentially absorbing and scattering certain colors of the Sun, creates the blue sky and the red sunsets.

What you'll need:

• Fish tank filled (~3/4) with water
• Slide projector
• A few drops of milk (you can grab some from the Greenhouse Cafe downstairs)
• A stirrer
• Polarized filters

Procedure:

• Fill the fish tank (about 3/4 full) with clear, tap water. You can think of this as an analogy for the perfectly transparent terrestrial atmosphere.
• Shine the light from the slide projector horizontally into the center of the end of the fish tank. If you look through the fish tank at the slide projector, it is like you are looking through the Earth's atmosphere at the Sun. If you look at the beam of light from the side, it is as if we are looking at the blue sky away from the Sun. Record observation in the table (pdf or docx).
• Now, pour a drop or two of milk (slowly!) to introduce some scattering and mimic dust particles in the Earth's atmosphere. Again mark your observations in the table.
• Use the polaroid filter to observe the fish tank from the side and from the end. Note how the brightness changes as you look from farther and farther away. Take two polarizing filters (one in front of the other) and rotate one of them: what do you notice?
• Repeat the above steps by adding a few more drops.
• Finally, use the polarizing filters to observer the polarization of the particles in the Earth's atmopshere. Again, see how the brightness changes as you look at the sky farther and farther away from the Sun. Try holding two filters in front of each other and rotating them. What do you see?

Results:

• This experiment was purely visual so there are no quantitative results.
• Summarize the following:
  • What were the colors you observed for the "white" light when you observed the clear water vs. the water with increasing amounts of milk added? From edge-on and head-on?
  • Describe how all this can be understood, and compared to sunsets and dusty red stars, by a light scattering (Rayleigh Scattering) process in which blue light is scattered more easily than red light.
  • Describe why this also accounts for why the (clear) sky is blue?
  • Describe how polarization works? Is the blue light or red light more polarized?