Announcements Monday Sept 18

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AnnouncementsMonday Sept 18

• Exam 1
• In class this Wednesday
• Covers Chaps 1-5, 7 (not 6)
• Practice exam on web
• Today 5pm -6pm (here, LR70) review session
• 10 questions similar to practice exam
• Open book, lecture notes, calculators
• Nothing electronic allowed except calculators.
• Today
• Chap 7 (Light and radiation)

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Chap 7 Radiation and Matter
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Radiation Guiding Ideas

• First measurement of speed of light c 300,00
  km/s in vacuum, Roemer 1670 Jovian satellite
  timing over a year
• How is the light from an ordinary light bulb
  different from the light emitted by a neon sign?
  Continuous vs. line radiation
• How can astronomers measure the surface
  temperatures of the Sun, stars, planets? Wiens
  Law
• What is a photon? Quantum nature of light, energy
  prop. to wavelength (duality of wave, particle
  picture)
• How can astronomers tell what distant celestial
  objects are made of? Spectral lines
  fingerprints of elements
• What are atoms made of? Structure of atoms (Bohr
  model)
• How does the structure of atoms explain what kind
  of light those atoms can emit or absorb? Bohr
  model of quantized electron orbits
• How can we tell if a star is approaching us or
  receding from us? Doppler effect

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Roemer (1676) First measured speed of light using
observations of Jovian satellites

• In 1676, Danish astronomer Olaus Røemer
  discovered that the exact time of eclipses of
  Jupiters moons varied based on how near or far
  Jupiter was to Earth.
• This occurs because it takes varying amounts of
  time for light to travel the varying distance
  between Earth and Jupiter.

3108 km
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Light is electromagnetic radiation and is
characterized by its wavelength

• White light is composed of all colors which can
  be separated into a rainbow, or a spectrum, by
  passing the light through a prism.
• Visible light has a wavelength ranging from 400
  nm (blue) to 700 nm (red). (1 nm nanometer
  10-9 m)

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Wave Nature of Light 2-Slit Interference
This is the pattern one would expect if light had
wave-like properties.
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Interference in water waves
This is the pattern one would expect if light had
wave-like properties.
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Light type of Electromagnetic radiation
Electromagnetic radiation consists of oscillating
electric and magnetic fields. The distance
between two successive wave crests is called the
wavelength and is designated by the letter l.
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EM radiation varies from radio (longest
wavelength) to gamma rays (shortest wavelength)
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A dense object emits electromagnetic radiation
according to its temperature.

• WIENS LAW The peak wavelength emitted is
  inversely proportional to the temperature.
• In other words, the higher the temperature, the
  shorter the wavelength (bluer) of the light
  emitted.

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Blackbody (Thermal) Spectrum
Note that for the objects at the highest
temperature, the maximum intensity is at the
shorter wavelengths and that the total amount of
energy emitted is greatest.
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Temperature Scales
In the Kelvin scale, the 0 K point is the
temperature at which there is essentially no
atomic motion is called absolute zero. In the
Celsius scale, this point is 273º C and on the
Fahrenheit scale, this point is -460ºF.
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Wiens law and the Stefan-Boltzmann law

• Wiens law relates wavelength of maximum emission
  for a particular temperature
• lmax T constant ? lmaxT 2.9106 nmK
• Stefan-Boltzmann law relates a stars energy
  output, called ENERGY FLUX, F, to its temperature
• F sT4
• ENERGY FLUX is measured in joules per square
  meter of a surface per second and s 5.67 X 10-8
  W m-2 K-4..

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Wiens Law example

• Sun (surface)

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Stefan-Boltzmann Law example

• Earth radiates mainly in the infrared

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• PRS question
• Jupiter has a surface temperature of 120K and a
  blackbody spectrum which peaks at a wavelength of
  30 microns. Plutos blackbody spectrum peaks at
  60 microns. What is its surface temperature?
• 15K
• 30K
• 60K
• 120K
• 240K

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Spectral lines Unique for each element or
compound
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Mass Spectrometers user to detect explosives,
drugs
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Kirchhoffs laws Determine type of spectrum
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Continuous spectrum

• Law 1 A hot opaque body, such as a perfect
  blackbody, or a hot, dense gas produces a
  continuous spectrum -- a complete rainbow of
  colors with without any specific spectral lines.
  (This is a black body spectrum.)

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Emission line spectrum

• Law 2 A hot, transparent gas produces an
  emission line spectrum - a series of bright
  spectral lines against a dark background.

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Absorption line spectrum

• Law 3 A cool, transparent gas in front of a
  source of a continuous spectrum produces an
  absorption line spectrum - a series of dark
  spectral lines among the colors of the continuous
  spectrum.

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Kirchhoffs Laws
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Features of the Suns spectrum created by passing
sunlight through a prism.
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Emission Line Spectra of A Few Common Elements
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PRS question

• A dilute hot gas (such as a neon beer sign)
  emits
• Emission line spectrum
• Absorption line spectrum
• Continuous spectrum
• Absorption lines superposed on continuous
  spectrum
• Both emission and absorption spectra

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The Electromagnetic Spectrum
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Electromagnetic Radiation Radio Waves (TV, ?
1m)
Antenna size 1m
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• PRS question

• List the emission of red, green, and blue light
  in order of increasing wavelength
• Blue, green, red
• Red, green, blue
• Blue, red, green
• Green, red, blue
• Red, blue, green

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But, where does light actually come from?

• Light comes from the movement of electrons in
  atoms.

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Rutherfords Experiment (1915) Showed that Atoms
Are Largely Empty Space!
Alpha particles from a radioactive source are
channeled through a very thin sheet of gold foil.
Most pass through showing that atoms are mostly
empty space, but a few are rejected showing the
tiny nucleus is very massive.
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An atom consists of a small, dense nucleus
surrounded by electrons (Note Nucleus actually
much smaller)
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Bohr Model of Atom

• The nucleus contains protons and neutrons
• All atoms with the same number of protons have
  the same name (called an element).
• Atoms with varying numbers of neutrons are called
  isotopes.

• Atoms with a varying numbers of electrons are
  called ions.

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Orbits of electrons
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Spectral lines are produced when an electron
jumps from one energy level to another within an
atom.
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Doppler Effect
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Doppler Effect Caused by Motion
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Doppler Shift

• Red Shift The distance between the observer and
  the source is increasing.
• Blue Shift The distance between the observer and
  the source is decreasing.
• Dl wavelength shift, Df frequency shift
• lo wavelength if source is not moving
• v velocity of source
• c speed of light

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Doppler Shift Example

• A spacecraft on its way to Mars transmits a
  signal at 100 MHz (1 MHz 106 Hz). It is
  received on Earth at 99.99 MHz. How fast is the
  spacecraft moving and in which direction?

Since observed frequency is lower, the spacecraft
is moving away from Earth.
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PRS question (60 sec)

• Radar signals reflected from an asteroid are
  detected at a frequency of 100.10 MHz. The
  original transmitted signal was 100.00 MHz. At
  what speed is the asteroid moving away from the
  observer in km/s?
• (c 300,000 km/s)