TE Enriched Sample (E)

Photoelectric Effect 1 26 Comparing quantum theory with classical theory The table below summarizes how classical theory and quantum theory explain the photoelectric effect. classical wave theory Einstein’s quantum theory • energy builds up continuously • the rate of energy transfer depends on light intensity only • frequency plays no role • one photon for one emission • intensity = N × hf where N is the no. of photons (per second per m 2 ) and hf is the energy of each photon 1. condition for emission • any frequency ✗ • hf > φ (= hf 0 ) ✓ 2. KE of photoelectrons • increases with intensity ✗ • increases with hf ✓ • K max = hf − φ ✓ 3. emission time • delay for energy to build up ✗ • immediate ✓ 4. rate of emission • increases with intensity ✓ • increases with intensity ✓ D Importance of the photoelectric effect In modern theory of light, a photon has both wave and particle properties. It behaves as a wave when it travels through a double-slit, and as a particle when it is emitted or absorbed by a material. Einstein’s concept of photon involves two new ideas: • the particle nature of light: a light beam consists of a stream of photons. • the quantization of energy of light: the energy of light is emitted and absorbed only in discrete packets (photons). Each packet has energy E = hf . The photoelectric effect provides an evidence for these new ideas. It also provides a way to determine the value of the Planck constant h . The value of h can be found from the slope of V s – f graph (see Example 1.5). Table 1.3 Comparison between the classical wave theory and Einstein’s quantum theory ( ✓ means it agrees with the observation; ✗ means it does not agree with the observation.) Web links The phenomenon is called single photon interference. http://youtu.be/GzbKb59my3U An analogy: The pilot-wave dynamics of walking droplets (by MITMathLab) http://youtu.be/nmC0ygr08tE 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.