TE Enriched Sample (E)

Photoelectric Effect 1 34 Summary Key Ideas Photon • Light is both a wave and a particle 1. light wave = oscillating E -field and B -field 2. light beam = a stream of photons • Light behaves as 1. wave when it travels through slits. ( a it diffracts and interferes) 2. particles when it is emitted or absorbed by material ( a in whole number of photons). • Energy of a photon E hf hc m = = ( c = f m ) • Intensity of a beam (unit: W m - 2 ) beam rate intensity N hf photon # = 6 7 8 4 4 E E 1. N = no. of photons per second per m 2 2. intensity = rate of energy delivered per m 2 Photoelectric effect • Emission of electrons from a metal surface when light of sufficiently high frequency falls on it • Evidence of particle nature of light Einstein’s proposal • Light is emitted and absorbed only in whole numbers of quanta (photons). • Each photon has energy hf . • Each emission is the result of absorption of one photon. Photoelectron’s KE • Einstein’s photoelectric equation: hf K photon electron metal max eV hf s 0 z = + E D E : ; 1. conservation of energy 2. interaction of one photon with one electron • With wide range of KE from 0 to K max • Max KE of the fastest electrons K hf hf hf eV max s 0 z = - = - = remaining = • Work function φ and threshold frequency f 0 φ = hf 0 1. min. energy required to free an electron 2. property of the metal 3. independent of the incident light • Stopping potential V s K max = eV s 1. cut-off voltage (zero photocurrent) 2. stops all photoelectrons from reaching the anode 3. corresponds to the max KE of the fastest photoelectrons Photoelectron • Condition to occur f > f 0 or hf > z (= hf 0 ) • Immediate • Rate of electrons emitted from cathode depends on arrival rate of photons. • Rate of electrons reaching anode 1. gives the size of photocurrent ( I ∝ N ) 2. drops with increasing opposing voltage 3. dies out at stopping potential Graphs L 2 L 3 L 1 V s L 2 = 2 L 1 L 3 = 3 L 1 L : intensity V 0 same metal, f = constant photocurrent I f 3 f 2 f 1 f 1 < f 2 < f 3 V s(1) V s(2) V s(3) V 0 photocurrent I same metal, I 0 = constant metal a metal b metal c f (or hf ) independent of L V s(a) V s (or K max ) eV s = h ( f − f 0 ) V s(b) f 0(b) f 0(a) f 0(c) 0 f 1 f 2 f 3 f 1 < f 2 < f 3 photocurrent I same metal, L = constant assume equal ratio of photoelectrons to photons absorbed V s(1) V s(2) V s(3) V 0 DSE Level-up Companion A revision exercise is provided for helping Ss to grip the key points. See Level-up Task 71A . 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.