1. Today’s Learning Outcomes: Do now: Read p173-4: Modern Physics & Photoelectric Effect Tūrei, 8 Mahuru 2015 1)What is the photoelectric effect? 2)What were physicists expecting and what did they actually observe? 3)How did Einstein (and Planck) explain this?

2. Read page 108 and 109 Answer Question 6 and 8

4. Add colour, pictures, diagrams, keywords... But get it done today!

5. The occurs when shining light (usually UV) onto a piece of metal causes electrons to be given off. The photoelectric effect occurs when shining light (usually UV) onto a piece of metal causes electrons to be given off. Photo-cells: Thin glass tube (evacuated) Emitter – curved metal plate Collector μAμA LIGHT electron flow

6. Photoelectric Experiments What was expected: What was expected: Brighter light would cause electrons with more kinetic energy to be emitted Brighter light would cause electrons with more kinetic energy to be emitted What actually happened: What actually happened: Brighter light caused more electrons to be emitted, but there was no change in the amount of energy they had Brighter light caused more electrons to be emitted, but there was no change in the amount of energy they had

7. Photoelectric Experiments What was expected: What was expected: If very dim light was used, it would take some time before any electrons had absorbed enough energy to escape from the metal If very dim light was used, it would take some time before any electrons had absorbed enough energy to escape from the metal What actually happened: What actually happened: With UV light, even the faintest light caused some electrons to be emitted instantly With UV light, even the faintest light caused some electrons to be emitted instantly

8. Photoelectric Experiments What was expected: What was expected: The frequency (or colour) of the light used would not affect the energy of the emitted electrons. The frequency (or colour) of the light used would not affect the energy of the emitted electrons. What actually happened: What actually happened: The higher the frequency, the higher the energy of the electrons. Below a certain frequency, no electrons were emitted. The higher the frequency, the higher the energy of the electrons. Below a certain frequency, no electrons were emitted.

9. Photoelectric Experiments Einstein explained these results, using an idea suggested by Max Planck, that said electromagnetic radiation comes in fixed “packets” or quanta of energy called photons Einstein explained these results, using an idea suggested by Max Planck, that said electromagnetic radiation comes in fixed “packets” or quanta of energy called photons The amount of energy each photon has depends on the frequency of the radiation. The amount of energy each photon has depends on the frequency of the radiation.

10. Photoelectric Experiments Each photon has a fixed amount of energy given by: Each photon has a fixed amount of energy given by: h=Planck’s Constant = 6.63x10 -34 Js h=Planck’s Constant = 6.63x10 -34 Js This suggested that light behaved like a moving particle, rather than a wave This suggested that light behaved like a moving particle, rather than a wave