Understanding Quantum Physics: Photoelectric Effect & Energy Levels

1. Chapter 28 0 Quantum Physics

2. 28Quantum Physics Slide 28-2

3. Slide 28-3

4. Slide 28-4

5. Slide 28-5

6. The Photoelectric Effect Slide 28-12

7. Swimming Pool Analogy Slide 28-13

8. The Effect of Voltage Between Anode and Cathode Slide 28-14

9. Photons: Light Quanta If the photon has enough energy (greater than the work function) an electron is emitted. Slide 28-15

10. Checking Understanding • In the photoelectric effect experiment, why does red light not cause the emission of an electron though blue light can? • The photons of red light don’t have sufficient energy to eject an electron. • The electric field of the red light oscillates too slowly to eject an electron. • Red light contains fewer photons than blue, not enough to eject electrons. • The red light doesn’t penetrate far enough into the metal electrode. Slide 28-16

11. Answer • In the photoelectric effect experiment, why does red light not cause the emission of an electron though blue light can? • The photons of red light don’t have sufficient energy to eject an electron. • The electric field of the red light oscillates too slowly to eject an electron. • Red light contains fewer photons than blue, not enough to eject electrons. • The red light doesn’t penetrate far enough into the metal electrode. Slide 28-17

12. Checking Understanding • Monochromatic light shines on the cathode in a photoelectric effect experiment, causing the emission of electrons. If the intensity of the light stays the same but the frequency of the light shining on the cathode is increased, • there will be more electrons emitted. • the emitted electrons will be moving at a higher speed. • both A and B are true. • neither A nor B are true. Slide 28-18

13. Answer • Monochromatic light shines on the cathode in a photoelectric effect experiment, causing the emission of electrons. If the intensity of the light stays the same but the frequency of the light shining on the cathode is increased, • there will be more electrons emitted. • the emitted electrons will be moving at a higher speed. • both A and B are true. • neither A nor B are true. Slide 28-19

14. Checking Understanding • Monochromatic light shines on the cathode in a photoelectric effect experiment, causing the emission of electrons. If the frequency of the light stays the same but the intensity of the light shining on the cathode is increased, • there will be more electrons emitted. • the emitted electrons will be moving at a higher speed. • both A and B are true. • neither A nor B are true. Slide 28-20

15. Answer • Monochromatic light shines on the cathode in a photoelectric effect experiment, causing the emission of electrons. If the frequency of the light stays the same but the intensity of the light shining on the cathode is increased, • there will be more electrons emitted. • the emitted electrons will be moving at a higher speed. • both A and B are true. • neither A nor B are true. Slide 28-21

16. Example Problem • Exposure to ultraviolet can damage the skin, as anyone who has spent too much time in the sun knows all too well. For this reason, there are suggested limits for exposure to ultraviolet in work settings. These limits are wavelength-dependent. At a wavelength of 313 nm, the maximum suggested total exposure is 500 mJ per cm2 of skin; for 280 nm, the limit falls to 3.4 mJ per cm2 of skin. • What is the photon energy corresponding to each of these wavelengths? • How many total photons does each of these exposures correspond to? • Clearly, the shorter wavelength photons have a much higher probability of causing damage. Explain why you might expect this to be true. Slide 28-22

17. X-ray Diffraction: The Bragg Condition Slide 28-23

18. Diffraction Patterns for X Rays, Electrons, and Neutrons Slide 28-24

19. Matter Waves An electron beam passing through a double slit produces an interference pattern similar to that for light. Slide 28-25

20. The Particle in a Box The possible modes are reminiscent of those for the modes of a stretched string. The possible modes have different energies: Slide 28-26

21. Example Problem Electrons in molecules with long chains of carbon atoms can move freely along the chain. Depending on what atoms are at the end of the chain, the electrons may well be constrained to stay in the chain and not go beyond. This means that an electron will work as a true particle in a box—it can exist between the fixed ends of the box but not beyond. The particle in a box model can be used to predict the energy levels. Such molecules will show strong absorption for photon energies corresponding to transitions between energy levels. One particular molecule has a “box” of length 1.1 nm. What is the longest wavelength photon that can excite a transition? Slide 28-27

22. Energy Levels and Quantum Jumps Slide 28-28

23. Checking Understanding • What is the maximum photon energy that could be emitted by the quantum system with the energy level diagram shown below? The minimum photon energy? • 7.0 eV • 6.0 eV • 5.0 eV • 4.0 eV • 3.0 eV • 2.0 eV • 1.0 eV Slide 28-29

24. Answer • What is the maximum photon energy that could be emitted by the quantum system with the energy level diagram shown below? The minimum photon energy? • 7.0 eV • 6.0 eV • 5.0 eV • 4.0 eV • 3.0 eV • 2.0 eV • 1.0 eV Slide 28-30

25. Conceptual Example Problem: The Uncertainty Principle A neutron beam has an even smaller wavelength than an electron beam. Why don’t we use neutron microscopes instead of electron microscopes? Slide 28-31

26. Wave-Particle Duality Slide 28-32

27. The Dual Nature of a Buckyball Slide 28-33

28. Summary Slide 28-34

29. Summary Slide 28-35

30. Summary Slide 28-36

31. Additional Questions • Light falling on a photoelectric cell is causing a steady current in the cell. A filter that transmits red light is placed in front of the cell, and the current suddenly ceases. This is because • the filter has reduced the intensity of the light. • the filter has eliminated the highest-energy photons. • the filter has reduced the energy of the photons in the beam. • the filter has slowed down the photons in the beam. Slide 28-37

32. Answer • Light falling on a photoelectric cell is causing a steady current in the cell. A filter that transmits red light is placed in front of the cell, and the current suddenly ceases. This is because • the filter has reduced the intensity of the light. • the filter has eliminated the highest-energy photons. • the filter has reduced the energy of the photons in the beam. • the filter has slowed down the photons in the beam. Slide 28-38

33. Additional Questions • Which of the following phenomena is best explained by treating light as a wave? • The threshold frequency in the photoelectric effect • The emission of only certain wavelengths of light by an excited gas • The limited resolution of a light microscope • The quantization of energy levels for a particle in a box Slide 28-39

34. Answer • Which of the following phenomena is best explained by treating light as a wave? • The threshold frequency in the photoelectric effect • The emission of only certain wavelengths of light by an excited gas • The limited resolution of a light microscope • The quantization of energy levels for a particle in a box Slide 28-40

35. Additional Questions • Which of the following phenomena is best explained by treating light as a particle? • The limited resolution of a light microscope • The diffraction pattern that results when x rays illuminate a crystal • The threshold frequency in the photoelectric effect • The quantization of energy levels for a particle in a box Slide 28-41

36. Answer • Which of the following phenomena is best explained by treating light as a particle? • The limited resolution of a light microscope • The diffraction pattern that results when x rays illuminate a crystal • The threshold frequency in the photoelectric effect • The quantization of energy levels for a particle in a box Slide 28-42

37. Additional Example Problem Port-wine birthmarks can be removed by exposure to 585 nm laser light. Pulses are strongly absorbed by oxyhemoglobin in the capillaries in the birthmark, destroying them. A typical laser pulse lasts for 1.5 ms, and contains an energy of 7.0 J. • What is the power of the laser pulse? • How many photons are in each pulse? Slide 28-43