Summary of Key Ideas

The Nature of Light

Optics and Telescopes

Nonoptical Astronomy

WHAT DID YOU THINK?

  • What is light? Light—more properly “visible light”—is one form of electromagnetic radiation. All electromagnetic radiation (radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays) has both wave and particle properties.
  • Which type of electromagnetic radiation is most dangerous to life? Gamma rays have the highest energies of all photons, so they are the most dangerous to life. However, ultraviolet radiation from the Sun is the most dangerous electromagnetic radiation that we commonly encounter.
  • What is the main purpose of a telescope? A telescope is designed primarily to collect as much light as possible.
  • Why do all research telescopes use mirrors, rather than lenses, to collect light? Telescopes that use lenses have more problems, such as chromatic aberration, internal defects, complex shapes, and distortion from sagging, than do telescopes that use mirrors.
  • Why do stars twinkle? Rapid changes in the density of Earth’s atmosphere cause passing starlight to change direction, making stars appear to twinkle. Seen from space, stars do not twinkle.

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