TE Enriched Sample (E)

The quantum theory of light Enrichment Checkpoint 5 31 1.2 1. Monochromatic light of fixed intensity is shone onto a metal surface. The frequency of the light is increased. True or false: (a) There will be more photons fallen onto the metal surface. (b) The emitted electrons move faster on average. 2. A strong beam of sunlight falling on a photoelectric cell gives a steady current. A red filter (only allowing red light to pass) is placed in front of the cell, and the current suddenly becomes zero. This is because A. the filter has reduced the intensity of the light. B. the filter has eliminated the highest-energy photons. C. the filter has reduced the energy of the photons in the beam. D. the filter has slowed down the photons in the beam. 3. Monochromatic light of different frequencies is shone onto a metal surface. The graph on the right shows how the maximum KE of a photoelectron K max varies with the frequency f . F T B K max 0 f Photoelectric efficiency The photocurrent not only depends on the intensity of light but also the photoelectric efficiency of the metal surface. The photoelectric efficiency (also called photoyield) is the ratio of photoelectrons to incident photons . The earliest experiments on the photoelectric effect were performed with potassium/sodium cathode surfaces. Their efficiencies were only about 0.1% (i.e. one electron is emitted for every 1000 photons hitting the surface). Modern photocells often use a cesium cathode surface, and have efficiencies as high as 20%. Note that, for the same surface, the photoelectric efficiency also changes greatly with light frequency. Teaching notes The figure below shows photoelectric efficiency as a function of wavelength for three commercially available photosurfaces: s-1, s-4 and s-5. Sketch on this graph the result obtained if the experiment is repeated with (a) metals with smaller work functions. (b) metals with larger work functions. (c) light of higher intensity. 4. A photocell is illuminated with monochromatic light and the photocurrent I P is plotted against the voltage V applied across it. I P 0 V Sketch on the above graph the I P –V graph for light that has (a) the same frequency but a higher intensity. (b) a higher frequency. 5. Explain why no time delay is observed for electrons to be ejected from a metal even if the intensity of light shone on the metal is very weak? 250 0.01 0.1 1 10 s-5 photoelectric e ciency / % s-4 s-1 500 750 wavelength / nm 1000 1250 (c) (b) (a) (b) (a) Further questions 6. (a) What are the physical meanings of the slopes of the graphs in Q3 and on p.27? (b) Would their slopes and intercepts change if light intensity increases? If yes, how? (a) Q3: h ; p. 27: h / e ; (b) unchanged (Ref: DSE 2018 Q2a, 2021 Q2.4) Sample © United Prime Educational Publishing (HK) Limited, Pearson Education Asia Limited 2023 All rights reserved; no part of this publication may be reproduced, photocopied, recorded or otherwise, without the prior written permission of the Publishers.