20   19 A fine-beam tube is used for investigating properties of electrons. An electron beam is produced inside a spherical glass bulb. The bulb contains neon gas at a very low pressure. (a) The neon gas is at a pressure of 1.25 Pa and a temperature of 25 °C. Calculate the number N of neon atoms inside the bulb. bulb diameter = 16.0 cm (4) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. N = ...................................................... Turn over 21   *(b) Interactions between electrons and the neon atoms in the tube make the beam visible. Part of the spectrum of visible light produced by these interactions is shown. (Source: © MoFarouk/Shutterstock) Explain the process that results in the emission of this spectrum. Your answer should include reference to energy levels in atoms. (6) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. (Total for Question 19 = 10 marks)