Diffraction and interferenceIB MYP Physics: Subtopic test

1. 1
   A harbour is protected by a long concrete breakwater with a single gap 15 m wide. Ocean swell with a wavelength of about 20 m arrives at the breakwater with its wavefronts parallel to the wall.

   (a)

   Waves spread out into the harbour after passing through the gap. What is the name of this effect?

   [1 mark]

   • AReflection
   • BRefraction
   • CDiffraction
   • DAbsorption

   (b)

   The engineers want the waves to spread out as little as possible after passing through the gap. Which gap width would do this best?

   [1 mark]

   • A5 m
   • B15 m
   • C20 m
   • D100 m

   (c)

   Describe what happens to the waves as they pass through the gap, and state what happens to their wavelength.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Noise-cancelling headphones have a tiny microphone that detects the steady low hum of an aeroplane engine. A circuit then makes a loudspeaker inside each ear cup produce a second sound wave with the same frequency and the same amplitude as the hum, timed carefully against the engine noise.

   (a)

   When the engine sound and the second sound wave overlap, their displacements add together. What is this called?

   [1 mark]

   • ASuperposition
   • BReflection
   • CDiffraction
   • DRefraction

   (b)

   At one point two wave crests arrive at exactly the same moment. What is the result at that point?

   [1 mark]

   • AThe waves cancel and there is silence
   • BA larger displacement, called constructive interference
   • CA smaller displacement, called destructive interference
   • DThe waves bounce back the way they came

   (c)

   Explain how the headphones reduce the noise heard by the passenger.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A class uses a ripple tank to investigate how the width of a gap affects the spreading of water waves. A vibrating bar produces straight waves of wavelength 2 cm. Two barriers leave a gap in the middle, and the students change the width of the gap between trials. Beyond the gap they measure the angle through which the waves spread out from the straight-on direction. They find that a gap of 8 cm gives a spread of 15°, a gap of 4 cm gives 35° and a gap of 2 cm gives 80°.

   (a)

   Identify the independent variable, the dependent variable and one control variable in this investigation.

   [3 marks]

   (b)

   Describe the trend in the results and explain it using the wavelength. Suggest one way to make the conclusion more reliable.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A city council plans a 4 m high concrete barrier beside a motorway to reduce traffic noise for homes about 30 m behind it. Traffic makes a deep rumble from lorries (wavelength about 3 m) and a high whine from car tyres (wavelength about 0.1 m). The barrier would cost the equivalent of 15 million US dollars, would need farmland to be cleared, and would cast shadows over some gardens.

   (a)

   Explain why the barrier reduces the whine from car tyres much more than the rumble from lorries.

   [6 marks]

   (b)

   Discuss and evaluate whether the council should build the barrier.

   [6 marks]

   Total for question 4: 12 marks

End of questions