Photoelectric effect
Cambridge A-Level Physics (9702) · Unit 22: Quantum physics · 8 flashcards
Photoelectric effect is topic 22.2 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 22 — Quantum physics , alongside Energy and momentum of a photon, Wave-particle duality and Energy levels in atoms and line spectra. In one line: Photoelectric emission. This is when electrons (photoelectrons) are emitted from the metal surface.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 8 flashcards — 3 definitions, 3 key concepts and 2 calculations — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
What phenomenon occurs when electromagnetic radiation illuminates a metal surface
Photoelectric emission. This is when electrons (photoelectrons) are emitted from the metal surface.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Understand that shining electromagnetic radiation on a metal surface may cause photoelectrons to be emitted from it.
- Understand what threshold wavelength and threshold frequency mean, and use both terms.
- Explain photoelectric emission using the energy of a photon and the work function energy.
- Recall hf = Φ + ½mvₘₐₓ² and use it.
- Explain why the photoelectric current is directly proportional to the intensity, while the photoelectrons' maximum kinetic energy is independent of it.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Photoelectric effect
- › Explanation of the photoelectric effect must treat electromagnetic waves as discrete packets of energy called photons.
- › Since energy is inversely proportional to wavelength, the threshold wavelength is the maximum wavelength that can cause photoelectric emission.
What phenomenon occurs when electromagnetic radiation illuminates a metal surface?
Photoelectric emission. This is when electrons (photoelectrons) are emitted from the metal surface.
Define 'threshold frequency' in the context of the photoelectric effect.
Threshold frequency (f₀) is the minimum frequency of electromagnetic radiation required to cause photoelectric emission from a specific metal surface. Below this frequency, no electrons are emitted, regardless of intensity.
Define 'threshold wavelength' in the context of the photoelectric effect.
Threshold wavelength (λ₀) is the maximum wavelength of electromagnetic radiation that will cause photoemission. Wavelengths longer than λ₀ will not have enough energy to release electrons.
Explain the relationship between photon energy, work function, and photoelectric emission.
For photoelectric emission to occur, the photon energy (hf) must be greater than or equal to the metal's work function (Φ). The excess energy becomes the kinetic energy of the emitted photoelectron.
State the equation that relates photon energy (hf), work function (Φ), and the maximum kinetic energy (KEmax) of a photoelectron.
The equation is hf = Φ + KEmax, where h is Planck's constant, f is the frequency of the incident radiation, Φ is the work function, and KEmax is the maximum kinetic energy of the emitted electron.
Why is the maximum kinetic energy of photoelectrons independent of the intensity of the incident radiation?
The maximum kinetic energy is determined only by the frequency (energy) of individual photons and the work function of the metal, because each electron absorbs the energy of one photon only. Intensity affects the number of photons arriving per second, not the energy of each photon.
How is photoelectric current related to the intensity of incident radiation?
Photoelectric current is directly proportional to the intensity of the incident radiation. Increased intensity means more photons, leading to more electrons being emitted per unit time, thus a larger current.
A metal has a work function of 3.0 eV. What is the minimum frequency of light needed to eject photoelectrons from it?
Use hf₀ = Φ, where Φ = 3.0 eV = 3.0 × 1.60 × 10⁻¹⁹ J = 4.8 × 10⁻¹⁹ J. Therefore, f₀ = Φ/h = (4.8 × 10⁻¹⁹ J) / (6.63 × 10⁻³⁴ J s) ≈ 7.2 × 10¹⁴ Hz.
Review the material
Read full revision notes on Photoelectric effect — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 22 — Quantum physics
Photoelectric effect sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Photoelectric effect deck
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