Wave-Particle Duality (AQA A Level Physics)

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

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

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

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

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

Figure 2

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 observations A and B.

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

Table 1

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

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

The speed of the wheel is further increased.

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

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

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

Figure 3

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

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.

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, , which is measured by the microammeter.

Figure 5

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.

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.

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.

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 and the corresponding photocurrent . The same source of electromagnetic radiation is used throughout the investigation.

Table 2

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