Energy and momentum of a photon
Cambridge A-Level Physics (9702) · Unit 22: Quantum physics · 7 flashcards
Energy and momentum of a photon is topic 22.1 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 22 — Quantum physics , alongside Photoelectric effect, Wave-particle duality and Energy levels in atoms and line spectra. In one line: A photon is a quantum of electromagnetic radiation. It's a discrete packet of energy exhibiting wave-particle duality; it has zero mass and travels at the speed of light in a vacuum.
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 7 flashcards — 4 definitions, 1 key concept and 2 calculations — covering the precise wording mark schemes reward. Use the 4 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.
Photon
A photon is a quantum of electromagnetic radiation. It's a discrete packet of energy exhibiting wave-particle duality; it has zero mass and travels at the speed of light in a vacuum.
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 the particulate nature of electromagnetic radiation.
- Understand a photon as one quantum (a discrete packet) of electromagnetic energy.
- Recall E = hf and use it.
- Use the eV (electronvolt) as an energy unit.
- Understand that a photon carries momentum p = E/c, where E is its energy.
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 Energy and momentum of a photon
- › For force calculations, remember that reflection causes double the change in momentum compared to absorption.
What is a photon?
A photon is a quantum of electromagnetic radiation. It's a discrete packet of energy exhibiting wave-particle duality; it has zero mass and travels at the speed of light in a vacuum.
State the equation that relates the energy of a photon to its frequency.
E = hf, where E is the energy of the photon, h is Planck's constant (6.63 x 10⁻³⁴ Js), and f is the frequency of the electromagnetic radiation.
What is the electronvolt (eV) and how is it related to the joule (J)?
An electronvolt (eV) is the amount of energy gained (or lost) by a single electron when it passes through a potential difference of 1 volt. 1 eV = 1.60 x 10⁻¹⁹ J.
How is the momentum of a photon related to its energy and the speed of light?
The momentum (p) of a photon is given by p = E / c, where E is the energy of the photon and c is the speed of light in a vacuum (approximately 3.0 x 10⁸ m/s).
Explain how electromagnetic radiation exhibits particulate nature.
Electromagnetic radiation, while exhibiting wave-like behavior, also comes in discrete packets of energy (photons). This is demonstrated in phenomena like the photoelectric effect, where electrons are emitted when light of sufficient frequency shines on a metal surface, behaving as discrete particles delivering energy.
A photon has an energy of 4.14 eV. Calculate its momentum.
First convert eV to Joules: 4.14 eV * 1.60 x 10⁻¹⁹ J/eV = 6.624 x 10⁻¹⁹ J. Then, using p = E/c, p = (6.624 x 10⁻¹⁹ J) / (3.0 x 10⁸ m/s) = 2.21 x 10⁻²⁷ kg m/s.
A photon has a wavelength of 500 nm. Calculate its energy in electronvolts.
First, find the frequency using c = fλ: f = c/λ = (3.0 x 10⁸ m/s) / (500 x 10⁻⁹ m) = 6 x 10¹⁴ Hz. Then, E = hf = (6.63 x 10⁻³⁴ Js) * (6 x 10¹⁴ Hz) = 3.978 x 10⁻¹⁹ J. Finally, convert to eV: (3.978 x 10⁻¹⁹ J) / (1.60 x 10⁻¹⁹ J/eV) = 2.49 eV.
Review the material
Read full revision notes on Energy and momentum of a photon — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 22 — Quantum physics
Energy and momentum of a photon 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 Energy and momentum of a photon deck
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