Download Foundation 1 - Discovering Astronomy

Chapter 3
Light and Telescopes
What do you think?
• What is the main purpose of a telescope?
• Why do stars twinkle?
If you pass white light through a prism,
it separates into its component colors.
long wavelengths
short wavelengths
R.O.Y. G. B.I.V
spectrum
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
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.
Thomas Young’s interference experiment
Scottish physicist James Clerk Maxwell showed
mathematically in the 1860s that light must be a
combination of electric and magnetic fields.
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
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!
Not all EM radiation can penetrate Earth’s atmosphere.
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
A refracting
telescope
uses a lens to
concentrate
incoming
light
Similar to a
magnifying glass
A larger
objective lens
provides a
brighter (not
bigger) image
lenses
reverse
images
Three main functions of a telescope
• brighten
(called light gathering power)
• see fine detail
(called resolution)
and least important,
• magnify
magnification = (objective lens focal length / eyepiece lens focal length)
Refracting telescopes have drawbacks
• Spherical aberration
• Chromatic aberration
Special achromatic compound lenses and lens
coatings can often fix this aberration
Refracting telescopes have drawbacks
• Spherical aberration
• Chromatic aberration
• Sagging due to gravity
distorting the lens
• Unwanted refractions
• opaque to certain
wavelengths of light
Reflecting telescopes use mirrors to
concentrate incoming starlight
Newtonian Focus
Prime Focus
Cassegrain focus
coude’ focus
Astronomer’s face two major
obstacles in observing the stars
• Light Pollution from Cities
• Effects of Twinkling from Earth’s atmosphere
Tucson, Arizona in 1959 and 1980
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
Rapid changes in the density of
Earth’s atmosphere cause passing
starlight to quickly change
direction, making stars appear to
twinkle.
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
A Charge-Coupled Device (CCD)
Ordinary Photographs vs. CCDs
Matching 10-m, multiple
mirror Keck Telescopes in
Hawaii with adaptive optics
High above
Earth’s
atmosphere,
the Hubble
Space
Telescope
provides
stunning
details about
the universe
Observations at wavelengths other than visible
light are revealing previously invisible sights
Visible light image
radio wavelength image
Radio wavelength observations
are possible from Earth’s surface
The Very Large Array (VLA) in New Mexico
Observations at other wavelengths are
revealing previously invisible sights
UV
infrared
Ordinary
visible
Map of
Orion
region
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.
Self-Check
1: 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.