(b) Calculate the threshold frequency for sodium. threshold frequency Hz (3) (Total 5 marks) 6. (a) Explain what is meant by the term work function of a metal. (2) (b) In an experiment on the photoelectric effect, the maximum kinetic energy of the emitted photoelectrons is measured over a range of incident light frequencies. The results obtained are shown in the figure below. maximum kinetic energy of photoelectrons 0.5 1.0 1 .5 2.0 2.5 frequency of incident radiation/10" Hz (i) A metal of work function W is illuminated with light of frequency f. Write down the equation giving the maximum kinetic energy, Ex, of the photoelectrons emitted in terms of W and f. EK = (ii) Use the data in the figure to determine the work function of the metal.(iii) Determine the maximum kinetic energy of the photoelectrons when the frequency of the incident radiation is 2.5 x 1015 Hz. (6) (c) The experiment is repeated but with the light incident on a metal of lower work function. Draw a new line on the figure that results from this change. (2) (Total 10 marks) 7. (i) Calculate the longest wavelength of electromagnetic radiation that will cause photoelectric emission at a clean lithium surface. work function for lithium W = 4.6 x 10-19 J Longest wavelength = m (ii) Calculate maximum kinetic energy of the electrons emitted when electromagnetic radiation of frequency 8.5 x 1014 Hz is incident on the surface. Maximum energy = (Total 6 mark 8. When a clean metal surface in a vacuum is irradiated with ultraviolet radiation of a certain frequency, electrons are emitted from the metal. (a) (i) Explain why the kinetic energy of the emitted electrons has a maximum value. (ii) Explain with reference to the work function why, if the frequency of the radiation is below a certain value, electrons are not emitted.(iii) State a unit for work function. (1) (b) Light energy is incident on each square millimetre of the surface at a rate of 3.0 x 10-10 J s-1. The frequency of the light is 1.5 x 1015 Hz. (i) Calculate the energy of an incident photon. answer = (2) (ii) Calculate the number of photons incident per second on each square millimetre of the metal surface. answer = (2) (c) In the wave theory model of light, electrons on the surface of a metal absorb energy from a small area of the surface. (i) The light striking the surface delivers energy to this small area at a rate of 3.0 x 10-22 J S-1. The minimum energy required to liberate the electron is 6.8 x 10-19 J. Calculate the minimum time it would take an electron to absorb this amount of energy. answer = S (1) (ii) In practice the time delay calculated in part c (i) does not occur. Explain how this experimental evidence was used to develop the particle model for the behaviour of light. (2)