1. Photoelectric Effect – Learning Outcomes  Describe the photoelectric effect.  Demonstrate the photoelectric effect.  Discuss the nature of the photon.  Discuss the effect of intensity and frequency on the photoelectric effect.  Solve problems about the photoelectric effect.  HL: Discuss the concept of threshold frequency.  HL: Solve problems using Einstein’s photoelectric law.  Describe the structure and operation of a photocell.  Demonstrate the use of a photocell.  Give uses of photocells. 1

2. Photoelectric Effect  Light is known to liberate electrons from atoms.  To be liberated, electrons need to be given a minimum amount of energy, which frees them from their bond to the atom.  Any energy less than this will not free the electrons.  The photoelectric effect is the emission of electrons from the surface of a metal due to incident light of a suitable frequency. 2

3. Demonstrate the Photoelectric Effect 1.Place a zinc plate on an electroscope. 2.Charge the plate negatively by induction. 3.Shine a white light lamp onto the plate. 4.Note no effect. 5.Replace the white light lamp with a UV lamp. 6.Note that the leaves of the electroscope collapse. 7.The UV light causes electrons to be emitted from the zinc. 3

4. Demonstrate the Photoelectric Effect 4

5. Photon  Einstein later proposed that light existed not as a continuous wave, but as separate packets of energy he called quanta. 5  At the time, scientists couldn’t understand why frequency mattered instead of intensity.  Imagine a large wave being unable to capsize a ship, but frequent small waves doing the job. Einstein got his Nobel prize for this by Andrew Dunn – CC-BY-SA-2.0

6. Photon Energy 6 Higher Level

7. Einstein’s Photoelectric Law 7 Higher Level

8. Photocell  Photocells are electronic devices designed to take advantage of the photoelectric effect.  They use a cylindrical photocathode which will emit electrons when light of a suitable frequency falls on it.  The electrons flow towards the anode, creating a current which is proportional to the intensity. 8

9. Demonstrate a Photocell 1.Connect a photocell and galvanometer in series. 2.Shine a light source at the photocell and note the current. 3.Move the light source closer to the photocell and note the current. 4.Since intensity increases as the light source gets closer, the photocell produces a larger current. 9

10. Uses of Photocells  Photocells will turn on when light shines on them and turn off when in darkness. This allows them to be used in:  Burglar alarms,  Automatic doors,  Controlling burners in central heating,  Sound tracks in films. 10 by cmacd123 – public domain