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Modern Physics

Modern Physics. Chapter 27 Quantum Physics. Day 1 Learning Goals. Understand the relationship between wavelength and intensity for blackbody radiation Understand how Planck’s Hypothesis explained the relationship between wavelength and intensity for blackbody radiation.

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Modern Physics

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  1. Modern Physics Chapter 27 Quantum Physics

  2. Day 1Learning Goals • Understand the relationship between wavelength and intensity for blackbody radiation • Understand how Planck’s Hypothesis explained the relationship between wavelength and intensity for blackbody radiation

  3. Classical Theory on Radiation • Thermal Radiation • What is it? • How does it occur?

  4. Classical Theory on Radiation • Blackbody Radiation • What is it? • How does it work? • Graph of Intensity of BBR vs. Wavelength • The Ultraviolet Catastrophe

  5. BBR vs Wavelength

  6. Super Max ( Max Planck ) • 1858 – 1947 • He Provided the Explanation for the spectral distribution of Blackbody Radiation (1900 ) • Awarded the Nobel Prize in 1918

  7. History of Blackbody Radiation • http://www.electro-optical.com/bb_rad/bb_rad.htm • http://galileo.phys.virginia.edu/classes/252/black_body_radiation.html

  8. Planck’s Hypothesis • Proposed electric oscillators called resonators • Resonators are quantized ( En = nhf ) • Resonators emit/absorb energy in discrete units called quanta (photons) • The Birth of Quantum Physics

  9. Day 2Learning Goals • Understand how the photoelectric effect gives credence to the particle theory of light • Know how to use the work function to solve problems involving the photoelectric effect

  10. The Photoelectric Effect • When light is incident on certain metallic surfaces, electrons are emitted • Photoelectrons • Hertz • Einstein’s (1905) explanation

  11. Photoelectric Effect

  12. The Photoelectric Effect • Graph of photoelectric current vs. potential difference (DV) • Current dependent on intensity • Current dependent on, DV

  13. The Photoelectric Effect • Stopping Potential • KEmax = eDVs • Independent of the radiation intensity • Work Function • Cutoff Wavelength • lc = c/fc = hc/f

  14. The Photoelectric Effect • Wave Theory could not explain: • Cutoff Frequency • KEmax independent of intensity • KEmax increases with light frequency • Electrons are emitted instantaneously • Photon Theory accounts for: • f,the work function • KEmax = hf – f • Depends only on light frequency • 1 - 1interaction b/w photons and electrons • Linear relationship b/w f and KEmax

  15. Applications of the Photoelectric Effect • Photoelectric Cell • Street lights • Breathalyzer

  16. Day 3Learning Goals • Understand the nature and production of x-rays so you can: • Calculate the shortest l of x-rays that may be produced by electrons accelerated through a specified voltage

  17. X-RAYS • Wilhelm Roentgen first noticed them in 1895 while studying electrical discharges • Characteristics of x-rays • Traveled at or near the speed of light • Were not deflected by electric or magnetic fields

  18. X-RAYS • In 1912 Max von Laue suggested diffracting x-rays • Used atomic crystal lattice as a diffraction grating • determined the wavelength of x-rays to be about 0.1 nm

  19. X-RAYS • The Production of x-rays • Electrons are accelerated through a DV of several thousand volts • Electrons collide with a metal plate • X-rays are the energy emitted when the electrons are decelerated, but why are they decelerated? • Threshold voltage

  20. X-RAYS • Graph x-ray intensity vs. wavelength • Continuous broad spectrum dependent on the applied voltage. Why? • Characteristic spikes in the graph are dependent on the target material • lmin = hc/(eDV) Shortest wavelength radiation that can be produced

  21. Day 4Learning Goals • Understand the concept of Compton scattering so you can • Describe Compton’s experiment, state the results, and how these results are explained • Account for the increase of photon wavelength, and explain the significance of the Compton wavelength

  22. The Compton Effect • Compton’s Experiment • X-ray beam of a specified wavelength, l0, directed at a block of graphite • Result was the scattered x-rays had a longer wavelength, l, • Amount of energy reduction depended on the angle at which the x-rays were scattered • The change in wavelength, Dl, is the Compton Shift.

  23. The Compton Effect • Compton’s Explanation • Photon  particle collision similar to billiard ball collisions. Which means what? • Dl = l – l0 = h/(mec) *(1 – cos q ) • Compton Wavelength, h /(mec), is very small compared to visible light.

  24. Day 5Learning Goals • Understand the concept of DeBroglie wavelength so they can: • Calculate the wavelength of a particle as a function of its momentum • Describe the Davison-Germer experiment, and explain how it provides evidence for the wave nature of electrons

  25. Pair Production & Annihilaiton • Energy of a photon is converted completely into mass, pair production • Electron and positron are created from a photon • Energy, momentum, & charge are conserved

  26. Pair Production & Annihilaiton • Minimum energy required to produce a positron • hfmin = 2mec2 ( E = mc2) • Pair production cannot occur in a vaccum, but can only occur in the presence of a massive particle (nucleus)

  27. Pair Production & Annihilation • Pair Annihilation: electron-positron pair produce two photons • Momentum has to be conserved

  28. Davison-Germer Experiment • Double slit experiment shooting particles at a double slit

  29. Evidence to Support the Particle Theory of Light • Photoelectric Effect • Compton Effect • X-Rays • Pair Production & Annihilation

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