Photons and PhotoElectrons

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Photons and PhotoElectrons

• Kahuna

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The photon

• A particle of light
• A quantum of light energy
• The energy of a given photon depends on the
  frequency (color) of the light

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But light is also a wave!

• Travels at constant speed c in a vacuum.
• c lf
• c 3 x 108m/s
• l wavelength (m)
• f frequency (Hz)

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Calculating photon energy

• E hf
• E energy (J or eV)
• h Plancks constant
• 6.625?10-34 J s or 4.14 ?10-15 eV s
• f frequency of light (s-1, Hz)

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The electron-volt (eV)is an energy unit

• Useful on the atomic level.
• If a moving electron is stopped by 1 V of
  electric potential, we say it has 1 electron-volt
  (or 1 eV) of kinetic energy!

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Converting eV to Joules (J)

• 1 eV 1.602?10-19J

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Photoelectric Effect experiment
Collector (-)
Photo- Diode ()
At a certain voltage, Vs, the current cant flow
anymore!
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Anomalous Behavior in Photoelectric Effect

• Voltage necessary to stop electrons is
  independent of intensity (brightness) of light.
• Photoelectrons are not released below a certain
  frequency, regardless of intensity of light.
• The release of photoelectrons is instantaneous,
  even in very feeble light, provided the frequency
  is above the cutoff.

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Voltage current for different intensities of
light.
i
I3 gt I2 gt I1
V
Stopping potential is unaffected!
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Voltage versus current for different frequencies
of light.
f3 gt f2 gt f1
i
V
Stopping potential becomes more negative at
higher frequencies!
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Photoelectric Effect

• Ephoton Kmax Wo
• Ephoton hf (Plancks equation)
• Kmax maximum kinetic energy of electrons
• Wo binding energy or work function
• hf Kmax Wo

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Graph of Photoelectric Equation
hf Kmax Wo Kmax hf - Wo y mx b
Kmax
f
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Absorption Spectrum

• Photon is absorbed and excites atom to higher
  quantum energy state.

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Absorption Spectrum

• Absorption spectra always involve atoms going up
  in energy level.

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Emission Spectrum

• Photon is emitted and atom drops to lower quantum
  energy state.

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Emission Spectrum

• Emission spectra always involve atoms going down
  in energy level.

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Summary of Wave-Particle Duality

• Waves are particles and particles are waves

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Energy

• Particle
• E K U
• Photon
• E hf

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Momentum

• Particle
• p mv
• Photon
• p h/l

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Wavelength

• Photon
• l c/f
• Particle
• l h/p
• deBroglie wavelength

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Compton Scattering

• Proof of the momentum of photons.
• High-energy photons collided with electrons.
• Conservation of momentum.
• Scattered photons examined to determine loss of
  momentum.

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Davisson-Germer Experiement

• Verified that electrons have wave properties by
  proving that they diffract.
• Electron diffraction