Polarisation and diffractionEdexcel International A Level Physics: Subtopic test

1. 1
   A student shines light from a lamp through a sheet of polarising filter and then through a second sheet of the same material placed directly behind it. She looks at the light that emerges from the second sheet while rotating that sheet slowly in its own plane.

   (a)

   Which observation provides evidence that light is a transverse wave?

   [1 mark]

   • AIt can be plane polarised
   • BIt can be diffracted
   • CIt can be refracted
   • DIt can travel through a vacuum

   (b)

   The first sheet is fixed. The student rotates the second sheet through one complete revolution of 360°. How many times does the transmitted light fall to (almost) zero?

   [1 mark]

   • A1
   • B3
   • C2
   • D4

   (c)

   Explain why the light emerging from the first sheet is plane polarised.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student shakes the free end of a long rope randomly in all directions in the plane at right angles to the rope. The rope passes through a narrow vertical slot cut in a wooden board and then, further along, through a second board with a narrow slot that can be turned to any orientation.

   (a)

   What is true of the wave on the rope after it has passed through the first vertical slot?

   [1 mark]

   • AIt is unpolarised, because it started as a random shaking
   • BIt is plane polarised in the vertical plane
   • CIt is plane polarised in the horizontal plane
   • DIt is longitudinal, because it passes through the slot

   (b)

   The second slot is set horizontal. What happens to the wave after the second slot?

   [1 mark]

   • AIt is plane polarised in the horizontal plane at the original amplitude
   • BIt continues with its vertical oscillations at the original amplitude
   • CIt becomes a longitudinal wave
   • DIts amplitude falls to almost zero

   (c)

   The student repeats the demonstration with a long spring, pushing and pulling its end along the spring. Explain why a pair of slots has no effect on this wave however they are oriented.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In a ripple tank, straight water waves of wavelength 3.0 cm travel at 0.12 m s⁻¹ towards a barrier. The barrier has a gap whose width can be adjusted by moving two metal bars.

   (a)

   Describe how the waves behave after the barrier when the gap is 3.0 cm wide and when it is 30 cm wide.

   [3 marks]

   (b)

   Calculate the frequency of the waves. Use Huygens' construction to explain why the waves spread into the region behind the barrier when the gap is 3.0 cm wide.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A television transmitter radiates plane-polarised waves of wavelength 0.50 m, with the oscillations in the horizontal plane. A householder uses an aerial made of horizontal metal rods. A large building 30 m wide stands between the transmitter and some of the houses behind it.

   (a)

   Explain what is meant by plane polarisation and why only transverse waves can be polarised. Predict how the strength of the signal received by the aerial changes as it is rotated through 360° about the direction of the transmitter.

   [6 marks]

   (b)

   Use Huygens' construction to explain why some television signal reaches houses in the shadow of the building. Explain why a radio signal of wavelength 3.0 m would reach those houses more strongly than the television signal.

   [6 marks]

   Total for question 4: 12 marks

End of questions