1. Experimental evidence that light is a wave:

2. Shine light of particular wavelength/frequency/color on metal plate C Electron ejected from plate C (“photo-electron”) with certain kinetic energy Number of photons that reach plate A per second gives current (read on ammeter) Potential difference (“bias voltage”) between plates C & A adjusted until current is zero  no electrons reach plate A Use conservation of energy to determine maximum initial kinetic energy of ejected photo- electron Photoelectric Effect

3. ConcepTest #49: In a photoelectric effect experiment, you can adjust the frequency and the intensity of the incoming light. Consider the following choices: a) You want to increase the kinetic energy of the ejected electrons. What should you adjust? b) You want to increase the number of ejected electrons. What should you adjust? 1. Increase f 2. Increase I 3. Increase either 4. No effect Photo-Electric Effect Mysteries

4. Millikan’s photo-electric effect “data” Einstein’s Photoelectric Effect Solution h = Planck’s Constant h = 6.63 x 10 -34 J s = 4.14 x 10 -15 eVs hc = 1240 eV nm  VERY USEFUL Photon hypothesis – light is a PARTICLE

5. Interpreting Intensity

6. 1. Energy increases 2. Energy decreases 3. No change 4. Not enough info ConcepTest #50: You have a beam of light aimed at a surface of fixed area A, and can measure the amount of energy delivered to that area in some fixed amount of time. Consider the following choices: a) What happens if you decrease the wavelength? b) What happens if you reduce the number of photons per second?

7. More evidence for the particle nature of light Compton (Scattering) Effect Localized Photon interactions with Matter

8. Wave Nature of Particles: de Broglie’s hypothesis

9. ConcepTest #51: You want to construct a microscope to look at small objects. You could build the microscope so that it uses photons or electrons. You can produce photons with momentum 6.63 x 10 -26 J∙s/m = 4.14 x 10 -7 eV∙s/m. You can also produce electrons with the same momentum. Which would give you better resolution in your microscope? 1. Photons 2. Electrons 3. Both equally good 4. Neither any good