Summary of Key Ideas
The Nature of Light
Photons, units of vibrating electric and magnetic fields, all carry energy through space at the same speed, the speed of light (300,000 km/s in a vacuum, slower in any medium).-
From longest to shortest wavelengths, radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays are all the forms of electromagnetic radiation.
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Photons sometimes behave as particles, sometimes as waves.
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Visible light occupies only a small portion of the electromagnetic spectrum.
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The wavelength of a visible-light photon is associated with its color. Wavelengths of visible light range from about 400 nm for violet light to 700 nm for red light.
Optics and Telescopes
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A telescope’s most important function is to gather as much light as possible. When possible, it also resolves (reveals details) and magnifies an object.
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Reflecting telescopes, or reflectors, produce images by reflecting light rays from concave mirrors to a focal point or focal plane.
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Refracting telescopes, or refractors, produce images by bending light rays as they pass through glass lenses. Glass impurity, opacity to certain wavelengths, and structural difficulties make it inadvisable to build extremely large refractors. Reflectors are not subject to the problems that limit the usefulness of refractors.
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Earth-based telescopes are being built with active optics and adaptive optics. These advanced technologies yield resolving power comparable to the Hubble Space Telescope.
Nonoptical Astronomy
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Radio telescopes have large, reflecting antennas (dishes) that are used to focus radio waves.
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Very sharp radio images are produced with arrays of radio telescopes linked together in a technique called interferometry.
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Earth’s atmosphere is fairly transparent to most visible light and radio waves, along with some infrared and ultraviolet radiation arriving from space, but it absorbs much of the electromagnetic radiation at other wavelengths.
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For observations at other wavelengths, astronomers mostly depend upon space telescopes. Such satellite-based observatories are giving us a wealth of new information about the universe and permitting coordinated observation of the sky at many wavelengths.
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Charge-coupled devices (CCDs) record images on many telescopes used between infrared and X-ray wavelengths.
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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