Advanced Higher Physics

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Advanced Higher Physics

• Wave-Particle Duality

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History

• Unexplained phenomena observed in early 20th
  century
• Did not fit with classical wave or particle
  models for previous situations
• Lead to new physical theory Quantum Physics

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Photo-electric Effect 1

• photo-current flows in circuit when cathode is
  illuminated with e-m radiation of sufficiently
  high frequency, f
• Potentiometer used to provide stopping
  potential, reducing photo-current to zero
• Stopping potential relates to kinetic energy of
  photo-electrons emitted from surface of cathode

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Photo-electric Effect 2

• stopping potential (i.e. kinetic energy of
  photo electrons) found to be independent of
  intensity of radiation
• KE of photon electrons dependent on frequency of
  radiation
• below threshold frequency, f0, no photo
  electrons are emitted

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Photo-electric Effect 3

• Observations explained by Einsteins quantum
  theory of light
• Light travels as photons or wave packets
• Photon energy, E hf or E hc / ?
• where h Plancks constant
• f frequency
• c speed of light
• ? - wavelength

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Photo-electric Effect 4

• Kinetic energy of photo electrons given by
• KE h(f f0)
• where h Plancks constant
• f frequency of incident radiation
• f0 threshold frequency for cathode

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Compton Scattering 1

• 1920s experiments by Arthur Compton
• X-rays used on thin graphite sheets
• Some x-rays found to be scattered
• Scattered x-rays found to have longer wavelength
• Explained in terms
• of collisions between
• photons and electrons

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Compton Scattering 2

• Incident x-rays have frequency, f, and energy, E
  hf
• Scattered x-rays have frequency, f', and energy,
  E hf'
• Kinetic energy of electron after collison,
  recoil energy, KErecoil hf - hf'
• f' lt f , so wavelength of scattered x-rays, ? gt
  ?

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Compton Scattering 3

• Compton looked at linear momentum
• Combining Einsteins relativistic energy, E mc2
  and photon energy, E hc / ?
• We get photon momentum, p h / ?

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Compton Scattering 4

• Using conservation of energy and (relativistic
  momentum) Compton showed that
• So, ?? is dependent on scattering angle, ?.
• This cannot be explained by the wave model of
  light, only the particle model.

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De Broglie Wavelength 1

• De Broglie applied photon momentum equation, p
  h / ?, to particles
• Suggested that particles with momentum, p, could
  have wavelength, ?, given by, ? h / p
• Effect later observed using electron beams
• De Broglie wavelength exists for all moving
  bodies (normally undetectable)

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De Broglie Wavelength 2

• De Broglie wavelength is basis for
• Electron diffraction to examine crystal and
  molecular structures
• (including DNA)
• Electron microscopy (? 10-10 m, gives better
  resolution than light, ? 10-7 m)

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