AP Physics 2: Algebra-Based, AP exams, 12–15%

Unit 15: Modern Physics

Unit 15 introduces the quantum and nuclear physics that classical models cannot explain. Blackbody radiation and the photoelectric effect motivate the photon, atomic spectra motivate energy quantization, and nuclear decay and mass-energy equivalence close the course. Wave-particle duality is the unifying theme.

Position: 7 of 7 units
Published weighting: 12–15% in the official framework
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Physics 2: Algebra-Based. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Blackbody radiation and photons: The spectrum emitted by a hot object and the photon model of light with energy set by frequency.
The photoelectric effect: Threshold frequency, work function, and why the results cannot be explained by a wave model alone.
Atomic energy levels and spectra: Quantized levels, absorption and emission, and reading a spectrum as evidence of level spacing.
Wave-particle duality: Matter waves, the de Broglie relationship, and the experiments that reveal wave behavior in particles.
Nuclear structure and radioactive decay: Nucleons, isotopes, alpha, beta, and gamma decay, and half-life reasoning.
Mass-energy equivalence: Mass defect, binding energy, and energy release in fission and fusion.

Study tips

How to practice Modern Physics.

Section-specific guidance that follows from how this material is usually tested.

Practice the photoelectric graph in both directions: read the work function from it, and predict its shape from parameters.
Keep a short list of the observations that classical physics fails to explain, with the quantum resolution for each.
Balance nuclear equations regularly; the arithmetic is easy but the conventions need repetition.

Where this fits

Unit 15: Modern Physics is 7 of 7 units in the AP Physics 2: Algebra-Based guide, and it carries a published weighting of 12–15% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Physics 2: Algebra-Based guide Official exam information

Sample Modern Physics questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
What is the energy of a single photon of light whose frequency is 5.0 x 10^14 Hz, using Planck's constant h = 6.63 x 10^-34 J s?
1. A1.3 x 10^-48 J
2. B3.3 x 10^-20 J
3. C3.3 x 10^-19 J
4. D6.6 x 10^-34 J
Show answer
Answer: C. 3.3 x 10^-19 J
Photon energy is E = hf = (6.63 x 10^-34)(5.0 x 10^14) = 3.3 x 10^-19 J. The 1.3 x 10^-48 J choice divides h by the frequency instead of multiplying.
Key insight: Photon energy depends only on frequency, so brighter light of the same color simply delivers more photons.
Question 2Foundational
When a nucleus undergoes alpha decay, how do its atomic number and its mass number change?
1. AThe atomic number decreases by 2 and the mass number decreases by 4
2. BThe atomic number decreases by 4 and the mass number decreases by 2
3. CThe atomic number increases by 1 and the mass number stays the same
4. DThe atomic number stays the same and the mass number decreases by 4
Show answer
Answer: A. The atomic number decreases by 2 and the mass number decreases by 4
An alpha particle is a helium-4 nucleus carrying 2 protons and 2 neutrons, so the parent loses 2 from its atomic number and 4 from its mass number. The choice describing an atomic number increase of 1 with no mass change describes beta-minus decay instead.
Key insight: Balance nuclear equations by conserving both nucleon number and charge on each side.
Question 3Intermediate
Light made of photons with energy 4.0 eV strikes a metal surface whose work function is 2.5 eV. What is the maximum kinetic energy of the electrons that are ejected?
1. A0.63 eV
2. B1.5 eV
3. C2.5 eV
4. D6.5 eV
Show answer
Answer: B. 1.5 eV
The photoelectric equation gives maximum kinetic energy as photon energy minus work function: 4.0 eV - 2.5 eV = 1.5 eV. The 6.5 eV choice adds the work function instead of subtracting the energy needed to free the electron.
Key insight: Below the threshold frequency no electrons are emitted at all, no matter how intense the light is.
Question 4Intermediate
A radioactive isotope has a half-life of 8.0 days. What fraction of an initially pure sample of that isotope remains undecayed after 24 days?
1. A1/3
2. B1/8
3. C1/16
4. D1/24
Show answer
Answer: B. 1/8
24 days is exactly three half-lives, and each half-life leaves half of what was there, so the remaining fraction is (1/2)^3 = 1/8. The 1/3 choice divides by the number of half-lives instead of halving repeatedly.
Key insight: Radioactive decay is exponential, so equal time intervals always remove equal fractions, never equal amounts.
Question 5Advanced
A nuclear reaction converts 2.0 x 10^-29 kg of mass into energy. Using c = 3.0 x 10^8 m/s, how much energy is released?
1. A6.0 x 10^-21 J
2. B1.8 x 10^-13 J
3. C1.8 x 10^-12 J
4. D2.0 x 10^-29 J
Show answer
Answer: C. 1.8 x 10^-12 J
Mass-energy equivalence gives E = mc^2 = (2.0 x 10^-29)(3.0 x 10^8)^2 = (2.0 x 10^-29)(9.0 x 10^16) = 1.8 x 10^-12 J. The 6.0 x 10^-21 J choice uses mc rather than mc squared.
Key insight: Because c^2 is enormous, a tiny mass defect releases an energy far beyond any chemical reaction.

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Unit 15: Modern Physics