TE Enriched Sample (E)

Photoelectric Effect 1 Checkpoint 4 24 1. Are the following statements derived from the quantum theory of light about the photoelectric effect correct? (a) A photon can eject at most one electron from a metal surface. (b) A photon becomes stationary after its energy is absorbed by an electron. (c) The work function is the amount of energy required by each electron to escape from a metal surface. 2. True or false: Not all the photoelectrons can have the max KE, because electrons emitted from a metal surface collide with each other and lose some energy. correct incorrect incorrect F 3. Ametal has a work function of 1.9 eV. (a) Calculate the threshold frequency for the emission of photoelectrons to occur. (b) Calculate the max KE, in eV, of the photoelectrons when the metal is illuminated by light of wavelength 400 nm. (c) What will happen if light of wavelength 750 nm is used instead? 4.59 × 10 14 Hz 1.21 eV No photoelectrons will be emitted. C Explaining the photoelectric effect hf 0 = z At this frequency f 0 , a photon can provide an electron with just enough energy to fulfill the work function of the metal. In quantum theory, a light beam consists of a stream of photons. For a given frequency f , each photon carries energy hf . The frequency f controls the energy of each individual photon. Increasing the intensity of the beam only increases the total number N of photons arrived. Now, we shall see how the concept of photon can be used to explain the features of the photoelectric effect. Feature 1 ✓ Threshold frequency If the energy of a photon is smaller than the work function of the metal, an electron cannot get enough energy to escape by absorbing a photon. Therefore, below a certain threshold frequency, no photoelectron will be emitted. At the threshold frequency f 0 , even the fastest electrons get zero KE: K hf 0 max 0 z = - = Hence, – – – – – frequency f < f 0 frequency f > f 0 – – – – – photons photons Fig. 1.19 Feature 1: Photons are not energetic enough to cause the emission of electrons if f < f 0 . Simulation Use the simulation ‘Photoelectric effect’ to illustrate the features of the photoelectric effect by varying the intensity and frequency of the incident radiation. Further questions (d) Now the metal is used as the cathode of a photocell. If an opposing voltage of 1 V is applied across the photocell, what is max. KE of the photoelectrons reaching the anode? Assume the metal is shone by a 5 eV photon beam. 2.1 eV (Ref: DSE 2017 Q2.2) 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.