Wave-particle dualityAQA A-Level Physics: Subtopic test

1. 1
   A lecturer explains that the behaviour of electrons and of light cannot be described by waves alone or by particles alone.

   (a)

   Which observation provides evidence that electrons have wave properties?

   [1 mark]

   • AElectrons are deflected by a magnetic field
   • BElectrons have a constant charge-to-mass ratio
   • CElectrons are released immediately from a metal by high-frequency light
   • DA beam of electrons passing through a thin graphite film forms a pattern of rings

   (b)

   What is the de Broglie wavelength of an electron moving at 2.0 × 10⁶ m s⁻¹?

   [1 mark]

   • A3.6 × 10⁻¹⁰ m
   • B2.7 × 10⁹ m
   • C7.3 × 10⁻¹⁰ m
   • D1.8 × 10⁻¹⁶ m

   (c)

   Explain how the photoelectric effect suggests that electromagnetic waves have a particulate nature.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   In a vacuum tube, electrons are accelerated through a potential difference and pass through a thin sheet of graphite. They produce a pattern of concentric rings on a fluorescent screen.

   (a)

   The potential difference accelerating the electrons is increased. What happens to the rings?

   [1 mark]

   • AThe rings get larger
   • BThe rings stay the same size
   • CThe rings get smaller
   • DThe rings disappear

   (b)

   Protons are accelerated through the same potential difference instead of electrons. Compared with the electrons, what is the de Broglie wavelength of the protons?

   [1 mark]

   • ALarger, because protons have a larger mass
   • BSmaller, because protons have a greater momentum
   • CThe same, because they have the same kinetic energy
   • DThe same, because they have the same charge

   (c)

   Explain why the rings move closer to the centre when the accelerating potential difference is increased.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In an electron diffraction experiment an electron is accelerated from rest through a potential difference of 250 V in a vacuum.

   (a)

   Calculate the speed of the electron.

   [3 marks]

   (b)

   Calculate the de Broglie wavelength of the electron. The spacing between atoms in the target material is about 2.5 × 10⁻¹⁰ m. Deduce whether the electrons will show significant diffraction.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A physics society magazine is running an article on how ideas about light and electrons changed in the early twentieth century. Interference and diffraction experiments in the nineteenth century showed that light behaves as a wave. In 1905 Einstein used photons to explain the photoelectric effect. In 1924 de Broglie suggested that particles such as electrons have a wavelength h/p, and in 1927 electron diffraction was observed by Davisson and Germer and, independently, by G. P. Thomson. The results were published and examined by other physicists.

   (a)

   Explain how the photoelectric effect and electron diffraction together show that both light and electrons have wave properties and particle properties.

   [6 marks]

   (b)

   Evaluate the role of experimental evidence, publication and peer review in the acceptance of de Broglie's hypothesis.

   [6 marks]

   Total for question 4: 12 marks

End of questions