Nature of Light

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Not-For-Profit Public Foundation
Nature of Light
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Light is a form of energy. Energy can be
transported by waves. Use waves as a tool to
understand light.
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WAVE PARAMETERS

• Wavelength (l) is the length from crest to crest
  or trough to trough.
• Frequency (f) is the number of waves passing a
  point per second.
• Higher Frequency ? Shorter Wavelength
• Lower Frequency ? Longer Wavelength
• Speed of a wave (v) Wavelength x Frequency

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Light as a WAVE
As a wave, Light can be described as having
a Frequency Wavelength Speed
Wave Speed Wavelength x Frequency
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• What does a light wave look like?
• To answer this, must first know how light is
  produced.
• Start with an electron ? has an electric field
  (example is static cling or battery)

• Also a moving electron produces a magnetic field.

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• Accelerate an electron and both electric and
  magnetic fields are produced ? called an
  electromagnetic wave

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The Nature of Light

• Light is an electromagnetic wave that travels at
  the speed of light
• The speed of light is related to the waves
  frequency and wavelength
• c ?f 300,000 km/sec
• (or 186,000 mi/sec)

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• As a wave, light may have any wavelength (and
  frequency), but its speed remains constant.
• An electromagnetic wave with a long wavelength
  has a low frequency and one with a short
  wavelength has a high frequency.
• Visible light is an electromagnetic wave that has
  wavelengths between 400 and 700 nanometers!
  (That is, 300 billionths to 700 billionths of a
  meter!)

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• When you have your teeth x-rayed, the dentist is
  shining an electromagnetic wave with a wavelength
  more than 1,000 times shorter than visible light
• When you listen to the radio, your radio is
  picking up electromagnetic waves almost a million
  times longer than visible light, or about 0.1
  meters.
• The range wavelengths (or frequencies) of
  electromagnetic waves is called the
  electromagnetic spectrum.

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Courtesy Lawrence Berkeley National Laboratory
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The Spectrometer
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(No Transcript)
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Why does each element have its own fingerprint?
Interaction between light and atoms
What are atoms?
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Elementary Particles Mass and Charge
Particle Mass Charge
Proton 1.67 x 10-27 Kg 1.6 x 10-19 Coulomb
Neutron 1.67 x 10-27 Kg Neutral
Electron 9.11 x 10-31 Kg -1.6 x 10-19 Coulomb
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Build a Model of the Hydrogen Atom
Electron
Electron
Proton
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Bohr Model of the Atom and Energy Levels
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Bohr Model of the Hydrogen Atom

• Electrons Orbit a Proton
• Electrons exist only in discrete orbits and
  nowhere in between
• When electrons absorb a photon of the correct
  energy, the electron jumps to a higher energy
  level
• When the electron falls to a lower energy level,
  a photon of only one certain energy is emitted
• The emitted photons have a discrete energy,
  frequency or wavelength that we observe in the
  spectrum

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Sir William Herschel

• More to Light than Meets the eye
• Discovery of Infra-red (1800)
• (Also discovered Uranus in 1787)

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Herschel Experiment
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Herschel Exp
13.4oC
14.6 o C
IR MOVIE
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Spectra

• Continuous Spectrum
• Thermal Radiation
• (black body radiation)
• Atomic collisions result excitation of atoms and
  in emission of energy at multiple frequencies.
• Emission Spectrum
• Excitation of electrons
• High energy level
• Return to ground state produces emission of
  energy
• Energy produced proportional to drop

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Atomic and Molecular Mechanisms
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Interpretation of Spectra
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Atomic and Molecular Mechanisms
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Jupiter
Infrared
Visible
Radio
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Formation of 21cm Radio waves(1420 MHz)
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Visible
Radio at 21 cm
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Milky Way Galaxy as Seen in Different Parts of
the Spectrum
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Multi-spectral Astronomy
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w w w . p a r i . e d u
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Infrared Movie