(i) photoelectric effect, including a simple experiment to demonstrate…

The idea

The photoelectric effect is the emission of electrons from a metal surface when electromagnetic radiation of sufficiently high frequency is incident on it. In a simple demonstration, a clean zinc plate is fixed to the cap of a gold-leaf electroscope. When the plate is negatively charged, the gold leaf diverges; shining ultraviolet radiation on the plate causes the leaf to fall because electrons are emitted and the charge is neutralised. If the plate is positively charged, the leaf stays diverged because emitted electrons are attracted back. Radiation below the threshold frequency produces no emission, however intense it is, which supports the photon model rather than a wave model. The emitted electrons have a maximum kinetic energy given by hf = φ + Ek(max).

What earns the marks

• The photoelectric effect is the emission of electrons from a metal surface when electromagnetic radiation is incident on it.
• Emission occurs only when the frequency of the radiation is at or above a threshold frequency that depends on the metal.
• The intensity of the radiation affects the number of electrons emitted per second, not the maximum kinetic energy of each electron.
• The maximum kinetic energy of an emitted electron is given by hf = φ + Ek(max), where φ is the work function.
• In the zinc-plate demonstration, ultraviolet radiation discharges a negatively charged electroscope but not a positively charged one.
• The observations support the photon model, in which one photon interacts with one electron.

Common mistakes and how to put them right

• Believing that brighter light of any colour will eventually cause emission.

  Put it right: Below the threshold frequency no emission occurs regardless of intensity.

• Thinking that increasing intensity increases the maximum kinetic energy of emitted electrons.

  Put it right: It increases the number of electrons emitted per second instead.

• Confusing the work function with the threshold frequency.

  Put it right: The work function is an energy and the threshold frequency is the minimum frequency.

• Describing the electroscope observation without linking it to electron emission and charge neutralisation.

Exam tips

• State the threshold condition clearly: emission requires f ≥ f₀, where f₀ is the threshold frequency.
• Use hf = φ + Ek(max) and identify each term before substituting values.
• When describing the demonstration, say what happens to the gold leaf and explain why in terms of emitted electrons.
• Compare photon and wave explanations explicitly when the question asks why the wave model fails.