18 SECTION B Answer ALL questions in the spaces provided. 16 Read the extract and answer the questions that follow. In the 17th century there were two proposed theories to explain the refraction of light. Using a wave model, Huygens stated that light slows down when it passes from air to water. Using a particle model, Newton stated that light speeds up when it passes from air to water. Newton’s theory was more readily accepted until the speed of light in water was measured in the 19th century. In the early 20th century, Einstein used observations from the photoelectric effect to provide evidence for the particle model of light. Nowadays, both the wave model of light and the particle model of light are accepted, as each can be used to explain different aspects of the behaviour of light. (a) Give two reasons why Huygens’ theory for the refraction of light eventually became accepted. (2) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. (b) A ray of light travelling in air is incident on some water with an angle of incidence of 35°. The angle of refraction is 26°. Deduce whether this is consistent with Huygens’ statement about the speed of light as it passes from air to water. Your answer should include a calculation. (3) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. Turn over 19 (c) Diffraction and interference can be explained using the wave model of light. In an investigation to determine the wavelength of light from a laser, the light passed through a diffraction grating with 300 lines per millimetre. A diffraction pattern consisting of a series of bright dots was observed on a screen. The following data were recorded: distance between grating and screen = 2.00 m distance from central maximum to 2nd order maximum = 89.0 cm. Calculate the wavelength of light from the laser. (3) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. Wavelength = ....................................................... 20 (d) In a demonstration of the photoelectric effect, electromagnetic radiation is shone onto a clean metal surface. It can be shown that the metal loses negative charge when the radiation has a frequency above a certain threshold frequency. Explain how the particle model of light is consistent with this observation. (3) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. (e) In the 1920s, experiments demonstrating diffraction of electrons confirmed de Broglie’s work on the wave nature of particles. In one such experiment an electron had a momentum of 4.8 × 10−24 kg m s−1. Measurements confirmed that the de Broglie wavelength of the electron was 1.40 × 10−10 m. Deduce that these observations are consistent with the value of h given on the data sheet at the back of this exam paper. (3) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. (Total for Question 16 = 14 marks)