The photoelectric effect

In a photoelectric effect experiment, monochromatic light is incident on a metal surface and the stopping potential is measured.

Which change results in a greater stopping potential?

A photon with an energy of 3.8 × 10⁻¹⁹ J is incident on the metal surface.

Electrons are emitted from the surface with a maximum speed of 2.5 × 10⁵ m s⁻¹.

In a photoelectric experiment, the maximum kinetic energy E_k(max) of the emitted electrons and the frequency f of the incident radiation are measured.

A graph of the variation of E_k(max) with f has:

• a gradient p

• an intercept −q on the E_k(max) axis

• an intercept r on the f axis.

What is the value of the Planck constant?

In the early 1900s a contradiction between experimental results and theoretical predictions was termed the ultraviolet catastrophe.

Compare these experimental results and theoretical predictions. You may sketch and label a graph or graphs on Figure 4 as part of your answer.

Figure 4

A proposal was made that allowed a resolution of the contradiction.

Outline this proposal.

In two separate experiments, electromagnetic radiation of variable frequency f is incident on the surfaces of plates made from metals X and Y. The work function of X is greater than the work function of Y.

Which graph shows how the maximum kinetic energy E_k(max) of photoelectrons emitted from the surfaces of the plates varies with f ?

Monochromatic light is incident on a metal surface in a vacuum and photoelectrons are emitted from the surface. The photoelectric current I is the rate of flow of charge from the surface. The maximum kinetic energy of the photoelectrons is E_k(max).

The frequency of the light is then increased. There is no change to the rate at which energy is incident on the surface.

What happens to E_k(max) and I when the frequency is increased?

Photoelectrons are released when monochromatic light with a photon energy of 4.2 × 10⁻¹⁹ J is incident on a metal surface. The work function of the surface is 2.4 eV.

What is the maximum speed of the photoelectrons as they leave the surface?

In photoelectricity, V_s is the stopping potential.

Monochromatic light with a photon energy of 4.1 × 10⁻¹⁹ J is incident on a metal surface. The maximum speed of the photoelectrons released is 4.2 × 10⁵ m s⁻¹.

What is the work function of the metal?

Figure 1 shows an arrangement used to investigate the photoelectric effect.

A current is measured on the microammeter only when electromagnetic radiation with a frequency greater than a certain value is incident on the photoemissive surface.

Explain why the frequency of the electromagnetic radiation must be greater than a certain value.

The apparatus in Figure 1 is used with a monochromatic light source of constant intensity. Measurements are made to investigate how the current I in the microammeter varies with positive and negative values of the potential difference V of the variable voltage supply.

Figure 2 shows how the results of the investigation can be used to find the stopping potential.

Determine the number of photoelectrons per second leaving the photoemissive surface when the current is a maximum.

Explain why I reaches a constant value for positive values of V.

Explain why I decreases as the value of V becomes more negative.

The investigation is repeated with a different photoemissive surface that has a smaller value of the work function. The source of electromagnetic radiation is unchanged.

Discuss the effect that this change in surface has on the value of the stopping potential.

At the end of the 19th century new information was obtained about black-body radiation and the photoelectric effect. This information challenged classical physics theories.

In 1895, Wien and Lummer carried out experiments to measure black-body radiation accurately.

Figure 2 shows a typical black-body radiation curve of the type obtained by Wien and Lummer.

State what is meant by black-body radiation.

Describe how the predictions of classical theory compare with Wien and Lummer’s experimental results.

Annotate Figure 2 as part of your answer.

In 1900 Max Planck suggested a solution to the problems of the classical theory.

Outline the main aspects of his suggestion.

Planck’s suggestion was developed by Albert Einstein to explain the results of photoelectric effect experiments.

Discuss Einstein’s explanation of photoelectricity and its significance in terms of the nature of electromagnetic radiation.

In your answer you should

• describe two relevant observations made in photoelectric experiments

• explain the failure of classical physics to account for these observations

• include the main aspects of Einstein’s theory and how he explained the observations.

Light of frequency 2.0 × 10¹⁵ Hz is incident on a metal surface. The work function of the

Which statement is correct?

Photons of wavelength 290 nm are incident on a metal plate. The work function of the metal is 4.1 eV

What is the maximum kinetic energy of the emitted electrons?

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.

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 A, B and C. 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.

Which conclusion about the relationship between the work functions of A, B and C is correct?

Tick (✓) the correct box.

the same.

Which row in the table is correct?