Question

7 (a)* The Planck constant h can be measured in an experiment using light-emitting diodes LEDs) Each LED used in the experiment emits monochromatic light. Te wavelength λ of the emitted photons is detemined during the manufacturing process and is provided by the manufacturer When the p.d. across the LED reaches a specific minimum value suddenly switches on emitting photons of light of wavelength the LED andi are related by the energy equation eVi hh flying lead Fig. 7.1 LED nm l red 1.98 2.10 2 yellow 546 4 blue 468 3.02 Discuss how you could use the circuit of Fig. 7.1 to determine accurate values for n and how data from the table can be used graphically to determine a value for the Planck constant 161 156 02 Turn over COCR 2086 (b) A beam of ultraviolet light is incident on a clean metal surface. The graph of Fig. 7.2 shows how the maximum kinetic energy KEmas of the electrons ejected from the surface varies with the frequency fof the incident light. 60 40 10 20 15 20 10 10 Hz Fig.7.2 (i) Explain how the graph shown in Fig 7.2 cannot be explained in terms of the wave-model for electromagnetic waves. 121 (ii) Use data from Fig.7.2 to find a value of 1. the Planck constant Planck constant = J s 121 2. the threshold frequency of the metal Turn over H15602 20 3. the work function of the metal. work function = J 121 END OF QUESTION PAPER

Answer 1

9.11b) . 2 akob.eloch‰aLi 14 f-fo