1.2 The puzzling photoelectric effect History of the photoelectric effect Photoelectric effect and photoelectrons Some observations about the photoelectric effect Check-point 2 Failure of wave theory in explaining certain experimental results Check-point 3 1 2 3 Book E2 Section 1.2 The puzzling photoelectric effect

History of the photoelectric effect Before the theory of photoelectric effect being developed, scientists had several observations. 1887 Hertz: More sparks pass across the ring gap in the sunlight. Why? 1888 Hallwaches: A –ve zinc plate loses charge if exposed to UV light but not to white light. Charge is not lost if the plate is +ve. Book E2 Section 1.2 The puzzling photoelectric effect

History of the photoelectric effect Before the theory of photoelectric effect being developed, scientists had several observations. 1899 Thomson: The leaked charge is e–. I think if the metal is shone by UV light, e– will be emitted. 1900 Planck: I can explain a hot body’s radiation by assuming that a hot body emits energy in discrete packets proportional to its frequency. Book E2 Section 1.2 The puzzling photoelectric effect

History of the photoelectric effect Before the theory of photoelectric effect being developed, scientists had several observations. 1902 Lénárd: KE of the e– emitted depends on the light frequency but not the intensity! No e– is emitted if the frequency is too low. Classical wave theory cannot explain this. Book E2 Section 1.2 The puzzling photoelectric effect

History of the photoelectric effect Before the theory of photoelectric effect being developed, scientists had several observations. 1905 Einstein: Lénárd’s results can be explained by assuming that light of frequency f contains packets of energy hf. 1916 Millikan: My experiment proves that Einstein’s model is correct. Their work converged to the discovery of photon. Book E2 Section 1.2 The puzzling photoelectric effect

1 Photoelectric effect and photoelectrons The photoelectric effect can be observed using a metal plate (copper or zinc) and a gold leaf electroscope. Negatively charged metal plate put on the electroscope.  Gold leaf deflects.  Presence of electric charge Book E2 Section 1.2 The puzzling photoelectric effect

1 Photoelectric effect and photoelectrons However… when the metal plate is illuminated by a UV lamp, the gold leaf drops quickly. If the metal plate is illuminated by white light, the gold leaf does not fall. Expt 1a Photoelectric effect Book E2 Section 1.2 The puzzling photoelectric effect

Book E2 Section 1.2 The puzzling photoelectric effect Experiment 1a Photoelectric effect Put a metal plate (copper or zinc) on the top of the gold leaf electroscope. Charge the metal plate negatively by EHT. Illuminate the metal plate by a ray box. Observe any change in the gold foil. Book E2 Section 1.2 The puzzling photoelectric effect

Book E2 Section 1.2 The puzzling photoelectric effect Experiment 1a Photoelectric effect Illuminate the metal plate by an ultra-violet lamp. Observe any change in the gold foil. Video 1.2 Expt 1a - photoelectric effect Book E2 Section 1.2 The puzzling photoelectric effect

Book E2 Section 1.2 The puzzling photoelectric effect 1 Photoelectric effect and photoelectrons The gold leaf drops quickly when the metal plate is illuminated by UV radiation (but not by visible light).  –ve charge on the plate   e– are emitted from the metal surface when EM radiation of short wavelengths shines on it.  Photoelectric effect (e– emitted: photoelectrons) Book E2 Section 1.2 The puzzling photoelectric effect

2 Some observations about the photoelectric effect The metal plate is illuminated by lights of different frequencies and intensities. The p.d. between the two electrodes can be varied continuously. If photoelectrons are emitted, the ammeter will record current. Simulation 1.2 Photoelectric effect Book E2 Section 1.2 The puzzling photoelectric effect

2 Some observations about the photoelectric effect a Threshold frequency For a given metal, if frequency of incident light < some min. value  No photoelectron is emitted however strong the light intensity is. The min. f : threshold frequency Book E2 Section 1.2 The puzzling photoelectric effect

2 Some observations about the photoelectric effect b Rate of photoelectron emission and intensity of light When frequency of light > the threshold value  Photoelectrons are emitted almost immediately (no time delay). Rate of emission  light intensity current measured brightness Book E2 Section 1.2 The puzzling photoelectric effect

2 Some observations about the photoelectric effect c Stopping potential If the potential at the collector is made more –ve  Some photoelectrons lack KE to overcome the p.d. to reach the collector.  Current  Book E2 Section 1.2 The puzzling photoelectric effect

Book E2 Section 1.2 The puzzling photoelectric effect c Stopping potential The p.d. Vs just enough to stop the most energetic photoelectrons and cease the current  Stopping potential ∴ The max. KE Kmax of the photoelectrons emitted: Kmax = eVs Book E2 Section 1.2 The puzzling photoelectric effect

Book E2 Section 1.2 The puzzling photoelectric effect c Stopping potential The stopping potential  is independent of the intensity of light.  increases with the frequency of light. increasing light intensity increasing light frequency intensity = constant frequency = constant stopping potential stopping potential stopping potential stopping potential Book E2 Section 1.2 The puzzling photoelectric effect

Book E2 Section 1.2 The puzzling photoelectric effect Check-point 2 – Q1 When electromagnetic radiation with its __________ larger than a minimum value shines on a ______ surface, _____________ are emitted from the surface. This phenomenon is called the photoelectric effect. frequency metal photoelectrons Book E2 Section 1.2 The puzzling photoelectric effect

Book E2 Section 1.2 The puzzling photoelectric effect Check-point 2 – Q2 It is found that for a certain metal X, photoelectrons can just be emitted by shining the metal with weak green light. Determine if each of the following statements is correct. (a) Photoelectrons cannot be emitted by weak violet light. ( T / F ) Book E2 Section 1.2 The puzzling photoelectric effect

Book E2 Section 1.2 The puzzling photoelectric effect Check-point 2 – Q2 It is found that for a certain metal X, photoelectrons can just be emitted by shining the metal with weak green light. Determine if each of the following statements is correct. (b) Photoelectrons cannot be emitted by bright red light. ( T / F ) Book E2 Section 1.2 The puzzling photoelectric effect

Book E2 Section 1.2 The puzzling photoelectric effect Check-point 2 – Q2 It is found that for a certain metal X, photoelectrons can just be emitted by shining the metal with weak green light. Determine if each of the following statements is correct. (c) The maximum kinetic energy of the photoelectrons emitted by bright green light is larger. ( T / F ) Book E2 Section 1.2 The puzzling photoelectric effect

Book E2 Section 1.2 The puzzling photoelectric effect Check-point 2 – Q2 It is found that for a certain metal X, photoelectrons can just be emitted by shining the metal with weak green light. Determine if each of the following statements is correct. (d) The stopping potential for photoelectrons emitted by weak violet light is larger. ( T / F ) Book E2 Section 1.2 The puzzling photoelectric effect

3 Failure of wave theory in explaining certain experimental results If light is regarded as a wave  The energy contained in a wave depends on its amplitude (i.e. intensity) and not on its frequency.  Some results found in the photoelectric-effect experiment cannot be explained. Book E2 Section 1.2 The puzzling photoelectric effect

3 Failure of wave theory in explaining certain experimental results Prediction by considering light as a wave: If light of high enough intensity is directed on the metal surface  e– should be emitted after absorbing sufficient energy, no matter how low the frequency is. Book E2 Section 1.2 The puzzling photoelectric effect

3 Failure of wave theory in explaining certain experimental results But the truth is that: No matter how intense the light is, no e– is emitted if its frequency is lower than the threshold frequency. Book E2 Section 1.2 The puzzling photoelectric effect

3 Failure of wave theory in explaining certain experimental results Prediction by considering light as a wave: (2) e– needs time to accumulate enough energy from the light before leaving the metal surface.  A time delay between the arrival of light and the ejection of the first e– is expected. Book E2 Section 1.2 The puzzling photoelectric effect

3 Failure of wave theory in explaining certain experimental results But the truth is that: No such time delay is observed. Book E2 Section 1.2 The puzzling photoelectric effect

3 Failure of wave theory in explaining certain experimental results Prediction by considering light as a wave: (3) e– are expected to have higher max. KE when light of higher intensity is illuminated on the metal surface. Book E2 Section 1.2 The puzzling photoelectric effect

3 Failure of wave theory in explaining certain experimental results But the truth is that: The max. KE of the photoelectron is independent of the intensity of the light. Such puzzles were explained by Einstein in 1905. (See chapter 1.3.) Book E2 Section 1.2 The puzzling photoelectric effect

Book E2 Section 1.2 The puzzling photoelectric effect Check-point 3 – Q1 True or false: If light is considered wave, the emission of electrons in the photoelectric effect would not depend on the frequency of light used. ( T / F ) Book E2 Section 1.2 The puzzling photoelectric effect

Book E2 Section 1.2 The puzzling photoelectric effect Check-point 3 – Q2 True or false: If light is considered a wave, the energy of light could not be transferred to electrons in a metal. ( T / F ) Book E2 Section 1.2 The puzzling photoelectric effect

Book E2 Section 1.2 The puzzling photoelectric effect The End Book E2 Section 1.2 The puzzling photoelectric effect