Radiation and Spectroscopy

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Radiation and Spectroscopy

• The Nature of Light and Atoms

2
Announcements

• Starry Night Enthusiast Project is due Thursday
• Thursdays reading assignment
• Finish all of Chapter 5 (focus particularly on
  sections 5.4, 5.4, and 5.6)
• Pick up graded assignments
• grading issues

3
Mission update
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Todays topics

• Speed of light
• Lights wave-like properties
• Concept of a Spectrum
• The electromagnetic spectrum
• Wiens law
• Stefan-Boltzmann law
• Lights particle-like properties

5
Activity 1

• What is spectroscopy? Why is useful? What
  information can it give us?

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Why is studying Light important to Planetary
Scientists?

• Spectroscopy !
• Shape of spectrum Temperature
• Lines in spectrum Composition
• Imaging at a variety of wavelengths

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The speed of light

• Galileos experiments (early 1600s)
• Inconclusive
• Romers observations (1675)
• Demonstrated that light moves at a finite speed
• Fizeau-Foucault method (1850)
• Rotating mirror
• Very close to the correct answer
• Speed of light is often denoted with the symbol
  c
• c 3 x 108 m/s (in a vacuum)

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Romers Observations

• Meticulous observations of the eclipses of
  Jupiters moons
• He found that the observed eclipse times were
  different than those predicted and depended on
  the relative position of Jupiter and the Earth
• Attributed this to being due to the finite speed
  of light

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The Speed of Light depends on the medium in which
it is moving
The refractive index of a medium is the speed
of light in a vacuum, divided by the speed of
light in the medium For instance, water has a
refractive index of about 1.3 (depending on the
wavelength)
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Waves

• Traveling disturbance
• Energy moves a long way, but the carrier doesnt
• Characterized by
• Wavelength Distance between crests
• Amplitude Height of crest
• Frequency Number of crests per second
• Velocity Speed and direction of the wave

Speed of a wave is the wavelength times the
frequency v ? ? (v
speed, ? wavelength, ? frequency )
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Changing Waves
Interference

• Interference
• Waves add when they collide
• Can be constructive or destructive
• Diffraction
• Waves bend going around corners
• Doppler Effect
• Waves compress or expanded due to moving source
• Useful to determine speed of wave emitters

Diffraction
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Youngs double-slit experiment

• This famous experiment convincingly demonstrated
  the wave-like property of light

13
Newtons Experiment
Showed convincingly that Light is a SPECTRUM
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Electromagnetic Spectrum

• Light has many frequencies and wavelengths
• But all move at the same speed in a vacuum
• Blue light has a shorter wavelength than Red light

15
Activity 2

• Suppose that a transmitter onboard a
  spacecraft in orbit about Mars sends two
  different signals to Earth. One of the signals
  is a radio wave, and the other a highly
  collimated beam of visible light. Which signal
  will arrive at Earth first?

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Why is it called the Electromagnetic Spectrum?

• Because Electric and Magnetic Fields are what is
  waving in light

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Imaging Solar System Bodies at Different
Wavelengths
Uranus
Sun
? Visible ?
Near IR
Xrays
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False color images Cassini/VIMS scientists
created these images by mapping 3 separate IR
wavelengths into Red,Green,Blue to get this image
of Titan