Interference and Young's double-slit experimentAQA A-Level Physics: Subtopic test

1. 1
   Two loudspeakers are connected to the same signal generator, so they emit sound of the same wavelength, 0.50 m, in phase with each other. A listener stands at a point P that is 4.00 m from one loudspeaker and 4.75 m from the other.

   (a)

   What is the path difference at P, expressed in wavelengths?

   [1 mark]

   • A1.5 wavelengths
   • B0.75 wavelengths
   • C3.0 wavelengths
   • D0.5 wavelengths

   (b)

   What does the listener hear at P?

   [1 mark]

   • AA loud maximum, because the waves arrive in phase
   • BA loud maximum, because the path difference is more than one wavelength
   • CA minimum, because the waves arrive in antiphase
   • DThe same loudness as from one loudspeaker, because the waves do not interact

   (c)

   The two loudspeakers are now connected to two separate signal generators, each set to emit nominally the same frequency. Explain why the listener no longer hears a stable pattern of loud and quiet positions.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student shines light from a laser of wavelength 650 nm at a pair of narrow parallel slits whose centres are 0.40 mm apart. A fringe pattern forms on a screen 2.5 m from the slits.

   (a)

   What is the fringe spacing on the screen?

   [1 mark]

   • A0.41 mm
   • B4.1 mm
   • C41 mm
   • D8.1 mm

   (b)

   The screen is moved to 1.25 m from the slits and nothing else is changed. What is the new fringe spacing?

   [1 mark]

   • A8.1 mm
   • B4.1 mm
   • C1.0 mm
   • D2.0 mm

   (c)

   Explain why a laser must never be viewed directly along the beam, and state one other precaution the student should take.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student measures the wavelength of the light from a laser pointer using a double slit with a slit separation of 0.30 mm. The screen is 2.00 m from the slits. She measures the distance across ten fringe spacings on the screen as 46 mm.

   (a)

   Calculate the wavelength of the laser light.

   [3 marks]

   (b)

   The student replaces the laser with a white light source and a narrow single slit placed just in front of the double slit. Describe and explain the appearance of the pattern on the screen.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In 1801 Thomas Young showed that light from a single source, passed through two closely spaced narrow slits, produces a pattern of bright and dark fringes on a screen. At that time many scientists favoured Newton's particle model of light.

   (a)

   Explain how Young's double-slit experiment gives evidence that light behaves as a wave. Refer to coherence and path difference in your answer.

   [6 marks]

   (b)

   Evaluate the extent to which Young's experiment settled the question of the nature of light.

   [6 marks]

   Total for question 4: 12 marks

End of questions