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
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- understand that photoelectrons may be emitted from a metal surface when it is illuminated by electromagnetic radiation
- understand and use the terms threshold frequency and threshold wavelength
- explain photoelectric emission in terms of photon energy and work function energy
- recall and use hf = Φ + 2
- explain why the maximum kinetic energy of photoelectrons is independent of intensity, whereas the photoelectric current is proportional to intensity
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.
- › Specifically state that diffraction is evidence for the wave nature of moving electrons; it does not demonstrate particle nature.
- › Remember that energy levels in an atom are always represented as negative values to signify they are bound states.
- › Since energy is inversely proportional to wavelength, the threshold wavelength is the maximum wavelength that can cause photoelectric emission.
- › For force calculations, remember that reflection causes double the change in momentum compared to absorption.
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. Intensity affects the *number* of photons, 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 hf = Φ, where Φ = 3.0 eV = 3.0 * 1.602 x 10⁻¹⁹ J. Therefore, f = Φ/h = (3.0 * 1.602 x 10⁻¹⁹ J) / (6.63 x 10⁻³⁴ Js) ≈ 7.24 x 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
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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