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
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 electromagnetic radiation has a particulate nature
- understand that a photon is a quantum of electromagnetic energy
- recall and use E = hf
- use the electronvolt (eV) as a unit of energy
- understand that a photon has momentum and that the momentum is given by p = E / c
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
- › 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 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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