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Wave-Particle Duality (AQA A Level Physics): Exam Questions

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1a
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Figure 2 shows a narrow beam of electrons produced by attracting the electrons emitted from a filament wire, to a positively charged metal plate which has a small hole in it.

Figure 2

Electrons are emitted from a filament wire which has a potential of 0. These are all attracted to a positively charged metal plate with a hole in it. A beam of electrons passes through this hole.

Explain why an electric current through the filament wire causes the wire to emit electrons.

1b
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Explain why the filament wire and the metal plates must be in an evacuated tube.

1c
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The potential difference between the filament wire and the metal plate is 4800 V.

Calculate the de Broglie wavelength of the electrons in the beam.

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1d
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The beam is directed at a thin metal foil between the metal plate and a fluorescent screen at the end of the tube, as shown in Figure 3. The electrons that pass through the metal foil cause a pattern of concentric rings on the screen.

Figure 3

Diagram of an electron beam in a glass bulb, showing side and front views. Side view has a metal foil which causes the electron beam to spread out. These electron beams hit a screen, which shows rings. Front view shows the screen with its concentric rings.

The potential difference between the filament and the metal plate is increased.

State and explain the effect this has on the diameter of the rings.

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2a
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Maxwell’s theory suggested the existence of electromagnetic waves that travel at a speed of:

square root of fraction numerator 1 over denominator epsilon subscript 0 mu subscript 0 end fraction end root

Hertz later discovered radio waves and performed experiments to investigate their properties.

Figure 4 shows a radio wave transmitter and a detector. The wave is transmitted by a dipole aerial. The detector consists of a metal loop connected to a meter.

Figure 4

Diagram of a signal generator connected to a dipole, transmitting to a metal loop detector connected to a meter.

Explain how the detection of the wave by the loop demonstrates the magnetic nature of the radio waves.

2b
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Explain how the electric nature of the waves emitted by the dipole could be demonstrated.

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3a
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Figure 3 shows the main parts of a transmission electron microscope (TEM).

Figure 3

Diagram of an electron microscope showing an electron gun producing a beam of electrons. The beam is represented by two paths of electrons. These diverging paths pass through a condenser, making them parallel. The paths travel through a thin sample, then through an objective lens where the paths converge and cross over. They once again converge in the projector lens and cross over and diverge. They then hit a fluorescent screen.

What is the process by which electrons are produced in an electron gun?

  • Beta particle emission

  • Electron diffraction

  • Photoelectric effect

  • Thermionic emission

Choose your answer

3b
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The electrons in a particular TEM have a kinetic energy of 4.1 × 10−16 J. Relativistic effects are negligible for this electron energy.

Suggest, with a calculation, whether the images of individual atoms can, in principle, be resolved in this TEM.

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4a
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Figure 2 shows the apparatus Fizeau used to determine the speed of light.

Figure 2

Light is incident on an angled partially reflecting mirror. One beam goes to an observer, the other goes to a mirror M, in line with the beam to the observer but in the opposite direction and returns along the same line back to the observer. A rotating toothed wheel W is placed such that the beam of light to the mirror passes through the teeth of the wheel.

The following observations are made.

A: When the speed of rotation is low the observer sees the light returning after reflection by the mirror M.

B: When the speed of the wheel is slowly increased the observer continues to see the light until the wheel reaches a certain speed. At this speed the observer cannot see the light.

Explain these observations.

4b
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Table 1 shows data from Fizeau’s experiment at the instant when observation B is made.

Table 1

d, distance from M to W

8.6 km

f, number of wheel revolutions per second

12

n, number of teeth in the wheel

720

It can be shown that the speed of light c is given by the equation

c space equals space 4 d n f

Discuss whether the data in Table 1 are consistent with the present accepted value for the speed of light.

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4c
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The speed of the wheel is further increased.

Deduce the value of f when the observer would next be unable to see light returning from the mirror.

4d
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Explain how the nature of light is implied by Maxwell’s theory of electromagnetic waves and Fizeau’s result.

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5a
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Figure 5 shows a vacuum photocell in which a metal surface is illuminated by electromagnetic radiation of a single wavelength. Electrons emitted from the metal surface are collected by terminal T in the photocell. This results in a photocurrent, I, which is measured by the microammeter.

Figure 5

A circuit is shown with vacuum photocell at the top. This is a circle containing a metal surface, on which electromagnetic radiation is incident, and a terminal T. A microammeter is in series with the vacuum photocell. The circuit connects to a voltmeter which is in parallel with another circuit containing a resistor and a cell. The voltmeter is connected to the resistor via a sliding contact. This acts as a potential divider.

The potential divider is adjusted until the photocurrent is zero.

The potential difference shown on the voltmeter is 0.50 V

The work function of the metal surface is 6.2 eV

Calculate the wavelength, in nm, of the electromagnetic radiation incident on the metal surface.

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5b
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The intensity of the electromagnetic radiation is increased. No adjustment is made to the potential divider.

The classical wave model and the photon model make different predictions about the effect on the photocurrent.

Explain the effect on the photocurrent that each model predicts and how experimental observations confirm the photon model.

5c
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The potential divider in Figure 5 is returned to its original position so that a photocurrent is detected by the microammeter. The potential divider is then adjusted to increase the potential difference shown on the voltmeter.

Explain why the photocurrent decreases when this adjustment to the potential divider is made.

5d
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The apparatus shown in Figure 5 is used to investigate three different metal surfaces X, Y and Z.

Table 2 shows, for each of the three surfaces, a voltmeter reading V and the corresponding photocurrent I. The same source of electromagnetic radiation is used throughout the investigation.

Table 2

V/ V

I/ μA

Metal surface X

1.5

56

Metal surface Y

2.5

56

Metal surface Z

2.5

78

Which conclusion about the relationship between the work functions of X, Y and Z is correct?

  • X > Y > Z

  • X < Y < Z

  • Y > X > Z

  • Y < X < Z

Choose your answer

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