16 16 The Planck constant can be determined in a school laboratory using light emitting diodes (LEDs). An LED emits light when the potential difference (p.d.) across it is large enough to transfer sufficient energy to an electron to result in the emission of a photon. The electron must have energy greater than or equal to the photon energy. The minimum p.d. required to produce light from LEDs emitting different frequencies was measured by increasing the p.d. from zero until light was first seen. The graph shows the results. 4.00 4.50 5.00 5.50 6.00 6.50 7.00 7.50 Frequency / 1014 Hz 2.60 2.40 2.20 2.00 1.80 1.60 1.40 1.20 1.00 Minimum p.d. / V Turn over 17 (a) Determine the value of the Planck constant given by this graph. (4) ................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................................... Value of Planck constant given by graph = .................................................................... (b) There are two problems with using LEDs to determine the Planck constant: • when the p.d. is increased and the LED first emits light it is difficult to see • the LEDs do not emit a single frequency but also light of frequencies slightly above and below the recorded frequency. Discuss the extent to which these problems are consistent with obtaining a result from this graph for the Planck constant which is higher than the accepted value. (3) ................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................................... ................................................................................................................................................................................................................................................... (Total for Question 16 = 7 marks)