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The Dual Personality

The Dual Personality. Particle or Wave?. Evidence for Wave Behavior. Solution to Maxwell’s Equations Interference Diffraction Polarization explained Adequately explains geometric optics. Classical Wave Theory Cannot Explain. Photoelectric Effect Cavity or Blackbody Radiation

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The Dual Personality

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  1. The Dual Personality Particle or Wave?

  2. Evidence for Wave Behavior • Solution to Maxwell’s Equations • Interference • Diffraction • Polarization explained • Adequately explains geometric optics

  3. Classical Wave Theory Cannot Explain • Photoelectric Effect • Cavity or Blackbody Radiation • Compton Effect

  4. PHOTOELECTRIC EFFECT • Inelastic collision of light with electron in metal What does the classical theory of light predict?????

  5. Failure of Wave Theory • Cannot explain energy absorption • Wave theory predicts a time delay that is not observed • Cannot explain intensity - current behavior • Cannot explain cutoff frequency

  6. Photoelectric Effect Einstein 1905Phillip Lenard

  7. Resolution? • Nature of Atomic Spectra • Bohr Model of the Atom

  8. Einstein Theorizes • Using Max Planck’s idea light comes in quanta (or photons) • Quanta totally absorbed during collision with one electron • Intensity (and thus photelectric current) proportional to number of quanta • Energy differences due to energy needed to remove electron from metal(Work Function)

  9. Success! • Predicts cutoff frequency • Correctly predicts intensity behavior • Correctly predicts nature of photoelectric current

  10. An aside about calculating energies and

  11. Work Functions

  12. COMPTON EFFECT • Elastic collision of light with electrons at relativistic energies(Binding energy in metals can be ignored) • Physical Laws Applied • Conservation of Momentum • Conservation of Energy • Classical waves predict wrong result

  13. Black Body Radiation • First indication that classical physics was in trouble followed from the failure of the classical wave theory to explain the observed spectrum of thermal radiation • Thermal radiation is that type of E&M radiation emitted by all objects merely because of their temperature

  14. Compton Effect Experiment and Results Compton scattering from entire atom (see text)

  15. Photon Theory Applied • Incident photon; electron is at rest • Recoil electron energy is relativistic • also mass is treated as relativistic • ‘Recoil’ photon has modified energy • Apply Conservation Laws for an Elastic Collision

  16. Photon Theory Applied • Results of Theory Theoretical Predictions Match Experiment!

  17. CONCLUSIONS • Light is wavelike for some experiments; e.g., Young’s Double slit • Light is particle like for some experiments; e.g. Photoelectric effect • Light has a dual nature adequately explained by its origin • Light is a finite wave train that, if localized, will give all its energy to one interaction (photon); the classical wave is now explained as having an intensity that is proportional to the number of photons

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