10 12 Vibrations of a car engine cause a sound wave in air. (a) Describe how the displacement of air molecules causes pressure variations in the air. 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Turn over 11 (b) A silencer is a device fitted to a car to reduce the sound from the engine. Some sound passes through the silencer chamber and is reflected twice. Some sound passes straight through the chamber without being reflected. The simplified diagram shows the paths of the sound as it travels through the chamber. Sound leaving the chamber is a combination of sound waves from the two paths. The sound waves are in phase as they enter the chamber. sound from engine chamber l An engine produces sound with a frequency of about 140 Hz. Explain why, to reduce this sound, the length l of the chamber should be about 60 cm. speed of sound in air = 340 m s−1 (4) .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. .................................................................................................................................................................................................................................................. (Total for Question 12 = 7 marks) 12 *13 A student views a laptop screen through a polarising filter. Initially the screen appears normal brightness. He rotates the filter to 90° and observes that the screen appears dark. Explain what the student observes as he gradually rotates the filter to 180° and then to 270°. 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(Total for Question 13 = 6 marks)