Chapter 3 Light and Telescopes. What do you think? What is the main purpose of a telescope? Why do...
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Transcript of Chapter 3 Light and Telescopes. What do you think? What is the main purpose of a telescope? Why do...
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Chapter 3 Light and Telescopes
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What do you think?
• What is the main purpose of a telescope?
• Why do stars twinkle?
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If you pass white light through a prism, it separates into its component colors.
R.O.Y. G. B.I.V
spectrum
long wavelengths short wavelengths
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If white light contains all colors, how fast does it move?
• 186,000 miles per second
• 300,000 kilometers per second
• 3 x 108 m/s
• first successfully determined by Danish astronomer Ole Roemer in 1675
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But, what is light?
• In the 17th Century, Isaac Newton argued that light was composed of little particles while Christian Huygens suggested that light travels in the form of waves.
• In the 19th Century, Thomas Young demonstrated that light bends slightly around corners and acts like interfering waves.
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Thomas Young’s interference experiment
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Scottish physicist James Clerk Maxwell showed mathematically in the 1860s that light must be a
combination of electric and magnetic fields.
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It wasn’t until 1905 that our current understanding of the nature of light emerged.
Einstein showed that light sometimes behaves as particles and sometimes as waves.
Photon energy = Plank’s constant x speed of light / wavelength
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Visible light is only one type of electromagnetic
radiation emitted by stars
Each type of EM radiation travels at exactly the same speed - the speed of light!
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Not all EM radiation can penetrate Earth’s atmosphere.
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Different types of EM radiation require different types of telescopes
• A refracting telescope uses a lens to concentrate incoming light
• A reflecting telescope uses mirrors to concentrate incoming starlight
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A refracting telescope
uses a lens to concentrate incoming
light
Similar to a magnifying glass
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A larger objective lens
provides a brighter (not
bigger) image
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lenses reverse images
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Three main functions of a telescope
• brighten(called light gathering power)
• see fine detail(called resolution)
and least important,
• magnifymagnification = (objective lens focal length / eyepiece lens focal length)
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Refracting telescopes have drawbacks
• Spherical aberration
• Chromatic aberration
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Special achromatic compound lenses and lens coatings can often fix this aberration
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Refracting telescopes have drawbacks
• Spherical aberration• Chromatic aberration
• Sagging due to gravity distorting the lens
• Unwanted refractions• opaque to certain
wavelengths of light
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Reflecting telescopes use mirrors to concentrate incoming starlight
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Newtonian Focus
Prime Focus
Cassegrain focuscoude’ focus
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Astronomer’s face two major obstacles in observing the stars
• Light Pollution from Cities
• Effects of Twinkling from Earth’s atmosphere
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Tucson, Arizona in 1959 and 1980
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Earth’s atmosphere hinders astronomical research
Image of stars taken with a telescope on the Earth’s surface
Same picture taken with Hubble Space Telescope high above Earth’s blurring atmosphere
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Rapid changes in the density of Earth’s atmosphere cause passing
starlight to quickly change direction, making stars appear to
twinkle.
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Advanced technology is spawning a new generation of
equipment to view the universe
• CCDs (charge-coupled devices)
• Large telescopes on remote mountain tops– Mauna Kea in Hawaii– Cerro Pachon in Chile
• Adaptive Optics to counteract the blurring of Earth’s atmosphere
• Orbiting space observatories
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A Charge-Coupled Device (CCD)
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Ordinary Photographs vs. CCDs
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Matching 10-m, multiple mirror Keck Telescopes in
Hawaii with adaptive optics
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High above Earth’s
atmosphere, the Hubble
Space Telescope provides stunning
details about the universe
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Observations at wavelengths other than visible light are revealing previously invisible sights
Visible light image radio wavelength image
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Radio wavelength observations are possible from Earth’s surface
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The Very Large Array (VLA) in New Mexico
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Observations at other wavelengths are revealing previously invisible sights
UV
Ordinary visible
infrared
Map of Orion region
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What did you think?
• What is the main purpose of a telescope? A telescope is designed to collect as much light
as possible. It also improves resolution and magnifies images.
• Why do stars twinkle? Rapid changes in the density of Earth’s
atmosphere cause passing starlight to change direction, making stars appear to twinkle.
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Self-Check1: List the major regions of the electromagnetic spectrum in order of wavelength and give
common examples of each.
2: List the colors of the visible spectrum in order of wavelength.
3: Name the two main classes of telescopes and describe the physical laws that each uses to form images.
4: Describe how the focal length and diameter of a telescope influence its angular resolution, light-gathering power, and magnifying power.
5: Draw a refracting telescope and reflecting telescopes with Newtonian, Cassegrain, prime, and coude’ focus locations, showing the path of parallel light rays through each.
6: Compare the merits and deficiencies of the two major classes of telescopes.
7: Discuss the similarities and differences of radio telescopes and optical telescopes.
8: List the advantages of orbiting telescopes over Earth-bound telescopes in detecting electromagnetic radiation in each of the major spectral regions.
9: Identify examples of observations impossible from the ground that have been made by the Hubble Space Telescope.