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Properties of Light
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c = Round Trip Distance/Total Time
The Speed of Light Galileo’s Experiment A B c = Round Trip Distance/Total Time
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Ole Roemer (1675) Jupiter Earth
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Speed of Light From Roemer’s Experiment Modern Value
c = Diameter of Orbit/Time Delay Modern Value c = 299, km/sec
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Speed of Light Absolute constant Nothing travels faster
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Relative Velocities Our physical sense of velocities is relative
How we measure speed depends on how we are moving and how the object we are measuring in moving Vrel = 2v v v v Vrel = 0 v
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Try it with Light Sun c ½ c Intuition says you get ½ c Einstein says c
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Try it with Light Sun c ½ c Intuition says you get 1½ c
Einstein says c
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Nature of Light Particle? Wave? Newton
Christian Huygens in Newton’s day Thomas Young (1801) Light can diffract - must be a wave Transverse wave
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Types of Waves Transverse and Longitudinal
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Wavelength
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Electromagnetic Spectrum
Increasing Wavelength Visible Light
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The Visible Spectrum 400nm nm nm nm Wavelength means COLOR
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Frequency Number of complete events occurring in a period of time
Waves/second Examples Second hand on a clock? 1 cycle/minute = 1/60 cycle/sec US Presidential Electons 1 election/4 years = 1/4 election/year
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Frequency and Period Period = 1 Frequency
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Speed, Wavelength, Frequency
c = f
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Photon Energy E f E = hf
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Properties of Light Speed (c) Nothing travels faster Wavelength ()
Absolute constant Wavelength () Gamma rays, X-rays, UV, Visible, IR, Radio In visible wavelength means color Frequency (f) Number of complete waves per second Energy of a Photon (E) E = hf
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Basic Spectrograph Collimating Lens Imaging Lens Dispersive Element
Slit Dispersive Element Imaging Lens Recording Device
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The atmosphere can act like a prism
Mercury near Horizon The atmosphere can act like a prism
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Source must be HOT and DENSE
Continuous Spectrum Source must be HOT and DENSE
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Emisson (Bright Line) Spectrum
Source must be HOT and TENUOUS
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Absorption Spectrum
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Types of Spectra Continuous Emission Absorption
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Rutherford Scattering Experiments
Atom He “bullet”
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Hydrogen Atoms Simplest atom Most abundant atom
One proton, one electron Most abundant atom 90% of universe is hydrogen
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Planetary Model Since electron orbits the proton, a force exists. e-
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Planetary Model e- Force implies acceleration
Accelerating charges emit light Light carries energy (E = hf)
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Planetary Model p e- Electron moves closer to the nucleus since it requires less energy to be there.
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Planetary Model Atoms do not exist!!
But the electron is still accelerating Must still be radiating energy (light) Must move still closer to the nucleus Electron will spiral into and collide with the nucleus (in about 10-8 seconds) Atoms do not exist!!
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Bohr’s Hypotheses Stable electron orbits exist where the electron does not lose energy.
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Electrons can be here or here but not here p
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Bohr’s Hypotheses Transitions can occur between orbits so long as the electron ends up with the energy of the new level.
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Hydrogen Absorption Energy 2 3 4 5 6563 Å photon
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Hydrogen Emission Energy 2 3 4 5 6563 Å photon
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Hydrogen Atom 434 nm 656 nm 486 nm 410 nm -e -e -e -e -e 400 nm 700 nm
2 656 nm -e 1 -e +P 486 nm 410 nm -e -e 400 nm 700 nm
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Emission and Absorption Lines
Lines come from electron transitions Energy change either comes from (absorption) or is given to (emission) photon. E Photon energy Frequency E = hf Frequency 1/(Wavelength) f = c/l Wavelength means COLOR
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Hydrogen Spectrum 4 3 Energy 2 1 Brackett (Far IR) Paschen (IR)
Lyman (UV) 1 2 3 4 Balmer (VIS) Brackett (Far IR) Paschen (IR) Energy
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Aluminum Argon Calcium Carbon Helium Hydrogen Iron Krypton Magnesium Neon Nitrogen Oxygen Sodium Sulfur Xenon
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Emission & Absorption Spectra for any Element
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Review
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End of Light
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