Diffraction: The Spreading of Light (page 2)

Try New Approaches

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   1. Resolution is the capacity of an optical instrument to distinguish between two separate but adjacent light sources. The resolving power of a telescope is a measure of resolution and relates to the amount of diffr:action fringe (blurred edge) around the image produced by the optical instrument. The better the resolving power, the less diffraction fringe and the clearer the image. light from two stars that appear close to each other may overlap and appear as one star when viewed through a telescope with low resolving power. Demonstrate this by making a second hole in the card as close as possible to the first one. Repeat the experiment and observe how the light passing through the holes is unresolved (not separated).
   2. How does the size of a telescope affect diffraction? Find out by making a hole about the size of a pencil point in an index card. Make several other holes that are even smaller. Repeat the original experiment, comparing the amount of interference in each hole. The clearer the light that passes through the hole, the less diffraction, the less interference, and the greater the resolution. From your results, determine which would produce better resolution, a telescope with a large or small lens or mirror?

Design Your Own Experiment

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   1. Diffraction hinders viewing the stars through a telescope. But diffraction also helps separate light into its different wavelengths. A spectroscope is a device that breaks light into its component parts. Below are steps for building one. For more information, see Philip Harrington, Astronomy for All Ages (Old Saybrook, Cf: Globe Pequot Press, 1994), pp. 178–179.
      • Cover the end of a paper towel tube with aluminum foil.
      • With adult approval, use a knife to cut a narrow slit in the aluminum foil cover.
      • Cover the open end of the tube with a diffraction grating and secure the grating with tape. (Purchase a diffraction grating at a science novelty store or a science supply company.)
      • Close one eye and look through the grating with your other eye. Point the hole in the tube toward the sky. CAUTION: Do not aim the grating at or near the Sun because it can permanently damage your eyes. Observe the colors that the light breaks into. These colors are called the visible spectrum. These colors, all together, make up white light. Consult a physical science or physics textbook for more information.
   2. How does the light source affect the spectra produced? Discover this by using your spectroscope to look at different lights, such as a candle flame, an incandescent light, a fluorescent light, and a neon light.

Get the Facts

1. The different types of spectra include continuous, emission, brightline emission, and dark-line emission. To learn more about the differences between spectra, see Brian Jones, The Practical Astronomer (New York: Fireside Books, 1990), pp. 42–43.
2. The spectra of light from stars vary in the brightness of some colors. They also differ in the number and arrangement of dark lines caused by destructive interference. How can dark spectral lines reveal whether the star is moving toward or away from Earth? To find out, see Robin Kerrod, The Star Guide: Learning How to Read the Night Sky Star by Star (New York: Macmillan, 1993), pp. 7–71.

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