Then the nucleus pays its binding bill in mass. The tool below drills all four arguments.
WeSolve+ reads the whole document and writes the questions for you
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The tool below is a small browser-only tool and it is not WeSolve+: paste a few lines and text rules turn them into cards on the spot. The real app, the one that uses AI, is behind the link above.
Unit 15 quantum arguments, from your own notes
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The photoelectric effect is an argument, so learn it as one
Classically, brighter light should eject electrons eventually at any colour. Measured: below a threshold frequency nothing ejects however bright, and above it ejection is instant however dim. The photoelectric effect therefore prices light in photons of energy h f: frequency sets each packet's size against the work function, intensity only sets how many arrive. State the observation, then the conclusion.
Matter waves: de Broglie turns the tables
De Broglie assigns every moving particle a wavelength of h over momentum, and electron diffraction confirms it: particles interfere like waves when the slit matches the wavelength. The scale explains the classical illusion, everyday momenta make wavelengths absurdly small, and exam questions ask exactly that: why electrons diffract and baseballs do not.
Energy levels make spectra into fingerprints
Electrons in atoms occupy discrete levels, and light is emitted or absorbed only in jumps: each photon's energy equals the gap, so each element's spectrum is a fingerprint of its level ladder. Bigger jumps make bluer lines. Given a level diagram, count the possible jumps for the line count, and match the largest gap to the shortest wavelength: both are standard moves.
Nuclear decays follow conservation bookkeeping
Alpha decay emits a helium nucleus, dropping two protons and two neutrons; beta decay converts a neutron to a proton with an electron emitted, charge conserved by the books; gamma sheds pure energy from an excited nucleus. Decay equations are audits: mass number and charge must balance across the arrow, and the exam grades the audit, not memory of which isotope does what.
Half life and the mass defect close the books
Half life halves the remaining sample on a fixed clock, whatever the starting amount, so activity questions are powers of one half. Binding energy comes from the mass defect: a nucleus weighs less than its parts, and the difference, cashed through E equals m c squared, is the energy holding it together. Fission and fusion both run downhill toward tighter binding, releasing the difference.
What to photograph for Modern Physics
Your level diagrams and decay audits. Related: Unit 14, photo to quiz and pricing.
Sources used on this page
- College Board, AP Physics 2: Algebra-Based
- Photoelectric effect
- Radioactive decay
- Matter wave
- Emission spectrum
- Mass-energy equivalence
- Active recall
- Spaced repetition
- Testing effect
- Forgetting curve
- Generation effect
- Judgment of learning
- Metacognition
- Desirable difficulty
- Distributed practice
- Formative assessment
- Flashcard
- Cloze test
- Multiple choice
- Test (assessment)
- Educational assessment
- Advanced Placement
- Curriculum
- Study skills
- Study guide
- Note-taking
- Overlearning
- Instructional scaffolding
- Item analysis
- Mastery learning
| Classical claim | The evidence against | The replacement |
|---|---|---|
| Light is only a wave | Threshold frequency | Photons of energy h f |
| Matter is only particles | Electron diffraction | Wavelength h over momentum |
| Energy is continuous | Line spectra | Discrete level ladders |
| Mass is conserved alone | Mass defect | Mass and energy trade |
| Decay is arbitrary | Balanced equations | Charge and nucleon audits |
| Amounts fade smoothly | Fixed halving clock | Half life powers of one half |
What does AP Physics 2 Unit 15 cover?
Modern physics: the photoelectric effect, matter waves, atomic energy levels and spectra, nuclear decay, half life and mass energy equivalence.
What does the photoelectric effect prove?
Light arrives in photons: frequency sets each packet's energy against the work function, and intensity only sets how many packets arrive.
Why do electrons diffract but baseballs do not?
Wavelength is h over momentum: electron momenta are small enough for wavelengths near atomic spacings, everyday momenta make them immeasurably small.
How do I read an energy level diagram?
Each emission line is a downward jump: photon energy equals the gap, so the largest gap gives the shortest wavelength line.
What balances in a decay equation?
Mass number and charge across the arrow: alpha drops four and two, beta converts a neutron with an electron keeping the charge books straight.
Can I build questions from my own Unit 15 notes?
Yes. Whatever you upload sets the boundary. Photograph the pages or send the chapter as a PDF and the questions stay inside it.
Last updated: 2026-08-15