CIE A Level Physics
CIE A Level Physics Quantum Physics Exam Questions
Free CIE A Level Physics exam questions for Quantum Physics, with a mark scheme for every pack. Covers photons, photon energy and momentum, the photoelectric effect, wave-particle duality, de Broglie wavelength, energy levels and line spectra.
Quantum Physics exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About CIE A Level Physics Quantum Physics
Quantum physics is Topic 22 of the Cambridge International AS & A Level Physics (9702) syllabus. It introduces the photon as a quantum of electromagnetic energy with energy E = hf and momentum p = E/c. You use the photoelectric effect as evidence for the particle nature of light and electron diffraction as evidence for the wave nature of particles, applying the de Broglie equation. The topic also explains emission and absorption line spectra using discrete energy levels in atoms. It builds on Waves and Superposition. Examiners often ask why the wave theory fails to explain the photoelectric effect.
What you need to know
- Photons Electromagnetic radiation has a particulate nature: a photon has energy E = hf = hc/λ and momentum p = E/c. The electronvolt (1 eV = 1.60 × 10^-19 J) is used for small energies.
- Photoelectric effect Photoelectrons are emitted only if the frequency is above the threshold frequency. Einstein's equation: hf = Φ + ½mvmax^2, where Φ is the work function.
- Intensity and frequency Maximum kinetic energy depends on frequency, not intensity. Increasing intensity increases the rate of photon arrival and so the rate of photoelectron emission (photoelectric current).
- Wave-particle duality Electron diffraction shows particles have wave properties. The de Broglie wavelength is λ = h/p.
- Energy levels Electrons in isolated atoms occupy discrete energy levels. A photon is emitted or absorbed when an electron moves between levels: hf = E1 - E2.
- Line spectra Emission spectra are bright lines on a dark background; absorption spectra are dark lines on a continuous spectrum, at the same frequencies for a given element.
Common mistakes in Quantum Physics
- Saying brighter light gives photoelectrons more kinetic energy. Intensity affects the number emitted per second, not their maximum energy.
- Forgetting to convert eV to J (or the reverse) when combining photon energy and work function.
- Saying all photoelectrons have the same kinetic energy. Only the maximum is given by Einstein's equation; others lose energy escaping.
- Using the speed of light in the de Broglie equation for an electron. Use the electron's momentum p = mv.
- Explaining line spectra as electrons emitting any energy. Only differences between discrete levels are possible.
Try three Quantum Physics exam-style questions
1. Ultraviolet light of wavelength 250 nm falls on a metal of work function 4.3 eV. Calculate the maximum kinetic energy of the photoelectrons in J. (3 marks)
Show the mark scheme answer
E = hc/λ = 6.63 × 10^-34 × 3.00 × 10^8/250 × 10^-9 = 7.96 × 10^-19 J (4.97 eV); KEmax = 4.97 - 4.3 = 0.67 eV; KEmax = 1.1 × 10^-19 J.
2. An electron is accelerated from rest through 150 V. Calculate its de Broglie wavelength. (3 marks)
Show the mark scheme answer
KE = 150 × 1.60 × 10^-19 = 2.4 × 10^-17 J; p = √(2mE) = √(2 × 9.11 × 10^-31 × 2.4 × 10^-17) = 6.6 × 10^-24 kg m s^-1; λ = h/p = 1.0 × 10^-10 m.
3. An electron in a hydrogen atom moves from the -3.4 eV level to the -13.6 eV level. Calculate the wavelength of the emitted photon. (3 marks)
Show the mark scheme answer
ΔE = 10.2 eV = 1.63 × 10^-18 J; λ = hc/ΔE = 6.63 × 10^-34 × 3.00 × 10^8/1.63 × 10^-18; λ = 1.22 × 10^-7 m.
Quantum Physics FAQs
What does Cambridge AS & A Level Physics Quantum physics cover?
It covers photon energy and momentum, the photoelectric effect, wave-particle duality and electron diffraction, the de Broglie wavelength, energy levels and line spectra.
Which paper is Quantum physics examined in?
It is an A Level topic assessed in Paper 4 (A Level Structured Questions).
What equations do I need for Quantum physics?
Learn E = hf, p = E/c, hf = Φ + ½mvmax^2, λ = h/p and hf = E1 - E2, and be fluent converting between eV and J.
