TE Enriched Sample (E)

The quantum theory of light 25 1.2 Using this result, we can rewrite Einstein’s photoelectric equation K hf max z = - as ( MC: DSE 2014 Q2.5, 2017 Q2.3, 2021 Q2.3) ( ) K h f f max 0 = - = eV s – − − − − − lower light frequency higher light frequency − − − − − photons photons Fig. 1.20 Feature 2: Photoelectrons are emitted with higher KE if the frequency of the light increases. where f is the frequency of the incident light and f 0 is the threshold frequency of the metal. Feature 2 ✓ Max. KE and frequency Since K hf max z = - , the maximum KE increases with the frequency f of the light. If the frequency of the light increases, each photon would have more energy. Thus, photoelectrons are emitted with higher KE. Changing the intensity of the light does not affect K max . It only affects the number of photons delivered per second and thus the number of photoelectrons emitted per second. Feature 3 ✓ Immediate emission All the energy of a photon will be transferred to an electron once it is absorbed. The electron will be emitted immediately if the photon’s energy is higher than the work function of the metal. There is no need to accumulate energy and there will be no time delay in the emission of photoelectrons. Feature 4 ✓ Emission rate and intensity Each electron in the metal to be emitted absorbs one photon and is emitted as a photoelectron. The number of photons delivered per second is proportional to the intensity. For light of a fixed frequency f > f 0 , the electrons are emitted at a higher rate when the intensity is increased. ( SQ: DSE 2013 Q2bii, 2020 Q2ai) − − – − − − lower light intensity higher light intensity − − − − − photons photons Fig. 1.21 Feature 4: More photons are delivered if the intensity of light increases. Teaching notes Emphasize that an electron absorbs only one photon. It will not absorb two or more photons simultaneously. 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.