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A –Level Physics: Waves and Quanta: Polarisation and the Photoelectric Effect Objectives: 70. be able to use the equation intensity of radiation 92. understand that the absorption of a photon can result in the emission of a photoelectron 93. understand the terms threshold frequency and work function and be able to use the equation 94. be able to use the electronvolt (eV) to express small energies 95. understand how the photoelectric effect provides evidence for the particle nature of electromagnetic radiation. Additional skills gained: • Practical Planning • Integrating GCSE content Starter: Explanations Using standing waves as examples; explain constructive and destructive interference (6 marks) You have 10mins Polarisation What does the word ‘polarising’ mean? “restricting the vibrations of a transverse wave wholly or partially to a specific direction” Draw what you believe an EM wave looks like What would it look Challenge like ‘side on’? When waves come from a luminous source the waves will reach us in various directions so can be considered to look like ‘E’ shown on the diagram below Copy the diagram down and then explain what is happening. Use the words ‘restricting’ and ‘polarisation’ What commonly used object is made to have this function and why? Have a try! Grab one of the filters from the box and put it up to see through it to the window. 1) Turn it, does anything change? 2) Put a second one behind it and slowly turn it, what happens? 3) Why is this happening? Exam Practice: 1 Time for a history lesson Grab one of the text books and turn to page 274. Read with us through the history of wave particle duality 10m Gold-Leaf Experiment Draw this set-up in your notes and then we will add the charges! Gold-Leaf Experiment 1) In this experiment, the metal cap (and as such, the stem and gold leaf) were given a negative charge 2) This caused the gold leaf to ______ because _________________ 3) Different colours of light were then shone onto the metal cap 4) Red light had no effect, but ultraviolet caused the gold leaf to go flat again Why is it important that the gold leaf be situated in a vacuum? Why did ultraviolet have this effect? [2 marks] Photoelectric Effect “the emission or release of electrons from a negatively charged metal when exposed to high frequency radiation” The energy of the incoming light, E = hf Where h= Plank’s constant and f= frequency One ‘packet of energy’ (photon) would allow the emission of one electron (photoelectron) Exam Practice: 2 5b) Explain why the following observations may be understood by using a photon model of light, rather than a wave model. • Light above a certain frequency causes the emission of electrons from the surface of a metal. This emission occurs instantaneously. • Light below a certain frequency will not result in the emission of electrons however long it illuminates the surface.(5 marks) .............................................................................................. ............................................... Work Function The electrons in different metals require different amounts of energy to escape. This is the work function of the metal. This is shown by the symbol ‘phi’ So: The energy coming in will be used to overcome work and the left over would be the kinetic energy of the particle! What would happen if the work function was greater than the ‘hf’? If I kept the type of metal constant, what could I do to increase the kinetic energy of the electrons emitted? Kinetic Energy You could increase the intensity of incoming light by moving it closer to the metal. (Intensity=power/area). What effect would this have on the energy of the electron emitted? So what would it affect? Increase intensity= increase NUMBER of electrons emitted Increase frequency= increase the KINETIC ENERGY of the electrons Calculating Work Function REMINDER! What was an ‘electronvolt’? It is “the amount of energy an electron gains after being accelerated by a p.d. of 1V.” eV Charge on a single electron Voltage NOTE: to convert joules into electronvolts, divide joules by the charge of an electron, which is about 1.602×10−19 Exam Practice: 3 An electron is accelerated from rest through a potential difference of 5.0 kV. The kinetic energy gained by the electron is A 8.0 × 10−16 J B 8.0 × 10−19 J C 3.2 × 10−20 J D 3.2 × 10−23 J Phototube A phototube consists of a anode that is radiated to release electrons that would hit the cathode and read as current Using a variable resistor, a p.d can be created across the system in the reverse direction. This would result in work being done against the flow from anode to cathode Phototube The p.d is increased until no current is recorded. thus the Work done=K.Emax. This is the stopping voltage (Vs) We do this to find out the kinetic energy of the emitted electrons! 1 2 eVs = mv2 Phototube So with our new value of kinetic energy (Ek) we go back to our original equation! Rearrange this equation into ‘y=mx+c’ DON’T FORGET! When you’re dealing with the KE, you use joules! Phototube So on a graph, the gradient would represent plank’s constant and the y-intercept would represent a negative value of the WORK FUNCTION! (phi) The threshold frequency is the mimumum frequency of incident radiation that would cause an electron to be emitted. It would be when the work function= incoming energy Exam Practice: 4 3. When electromagnetic radiation is incident on a metal plate, electrons may be emitted. (a) State what is meant by threshold frequency. (1) .................................................................................................... ......................................... (b) Calculate the threshold frequency for a metal with a work function of 2.28 eV. Charge on an electron=1.6x10-19C and Plank’s constant = 6.63×10−34Js (3) Threshold frequency = .................................... CLICK FOR ANSWER Exam Practice: 5 Ultraviolet radiation of wavelength 2.00 × 10−7 m is shone onto a zinc plate. Calculate the maximum speed of the electrons emitted from the plate. work function of zinc = 6.88 × 10−19 J (4) Maximum speed of electrons = ........................................... CLICK FOR ANSWER Independent Study .