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Unit 7 · Modern Physics Flashcards Cheat Sheet Essentials Visual Review MC Practice FRQ Practice

AP Physics 2 Unit 7 Visual Review

A topic-by-topic visual walkthrough of Unit 7: Modern Physics — wave-particle duality, the Bohr model, spectra, the photoelectric effect, Compton scattering, and nuclear physics.

← Back to Unit 7 hub
TOPIC 7.1 Quantum Theory & Wave–Particle Duality Light & matter are both E = h f λ = h / p Light behaves as a WAVE (interference) and as PARTICLES called photons. Matter (electrons) also shows wave behavior — the de Broglie wavelength. Photon energy example h = 6.63×10⁻³⁴ J·s (Planck's constant). Green light, f = 5.5×10¹⁴ Hz: E = hf ≈ 3.6×10⁻¹⁹ J Also E = hc/λ. Energy is QUANTIZED — delivered in whole photons. Which behavior shows up? Interference/diffraction reveal the wave side; the photoelectric effect & Compton scattering reveal the particle side. Light & matter are both wave and particle: E = hf, λ = h/p. The Review Hub · AP Physics 2 · Unit 7 TOPIC 7.2 The Bohr Model of the Atom photon electrons occupy fixed orbits Energy levels are quantized Electrons exist only in specific allowed orbits, each with a definite energy Eₙ. Jumping DOWN a level emits a photon; jumping UP absorbs one. ΔE = E_high − E_low = h f Hydrogen: Eₙ = −13.6/n² eV. Only photons matching a gap ΔE interact — this explains discrete spectral lines. Electrons occupy quantized levels; a jump gives ΔE = hf. The Review Hub · AP Physics 2 · Unit 7 TOPIC 7.3 Emission & Absorption Spectra EMISSION: bright lines on black ABSORPTION: dark lines on rainbow lines fall at the SAME wavelengths A fingerprint of each element EMISSION: excited atoms drop levels and emit photons at specific wavelengths → bright lines. ABSORPTION: atoms absorb those exact photons from white light → dark lines at the same places. Line positions come from the ENERGY-LEVEL gaps: ΔE = hf = hc/λ. Used to identify stars' & gases' composition. Each element's line spectrum is a fingerprint set by its energy gaps. The Review Hub · AP Physics 2 · Unit 7 TOPIC 7.4 Blackbody Radiation hotter cooler intensity vs. wavelength Iλ Hot objects glow Every warm object radiates a continuous spectrum whose shape depends only on its temperature. HOTTER → peak shifts to SHORTER λ (red-hot → white-hot → blue). λ_peak ∝ 1 / T (Wien) Explaining this curve required Planck to QUANTIZE energy — the birth of quantum theory. Hot bodies radiate a temperature-set spectrum; hotter → shorter peak λ. The Review Hub · AP Physics 2 · Unit 7 TOPIC 7.5 The Photoelectric Effect KE_max = h f − φ (φ = work function) metal surface photon ejected e⁻ Why it needs PHOTONS Light ejects electrons only if the photon frequency exceeds a THRESHOLD f₀. Below f₀: no electrons, no matter how bright. Brighter light = MORE electrons, not faster ones. Higher f (not intensity) → higher KE_max. KE_max = hf − φ; below threshold f₀ no electrons escape — proof light is quantized. The Review Hub · AP Physics 2 · Unit 7 TOPIC 7.6 Compton Scattering in: short λ out: longer λ recoil e⁻ photon–electron collision Photons carry momentum A high-energy photon (X-ray) collides with an electron like two billiard balls. photon momentum p = h / λ The photon gives energy & momentum to the electron, so it leaves with LESS energy → LONGER wavelength. Both energy AND momentum are conserved in the collision. Direct proof that light behaves as particles. Photons carry p = h/λ; scattering off electrons lengthens λ — light acts particle-like. The Review Hub · AP Physics 2 · Unit 7 TOPIC 7.7 Fission, Fusion & Nuclear Decay E = Δm c² (mass ↔ energy) Fission A heavy nucleus (e.g. U-235) SPLITS into smaller nuclei, releasing energy + neutrons. Powers reactors & chain reactions. Fusion light nuclei JOIN (powers the Sun). Mass–energy & conservation The products have slightly LESS mass than the reactants; the "missing" mass Δm becomes energy via E = Δmc². Nuclear reactions conserve charge (Z) and nucleon number (A) — balance both sides. This binding energy dwarfs chemical energies. Fission splits, fusion joins; lost mass → energy via E = Δmc² (conserve A & Z). The Review Hub · AP Physics 2 · Unit 7 TOPIC 7.8 Types of Radioactive Decay Alpha (α) Emits a helium nucleus (2p + 2n). A − 4, Z − 2 Heavy, +2 charge. Least penetrating — stopped by paper. most ionizing Beta (β) A neutron → proton, emitting an electron. A same, Z + 1 Light, −1 charge. Medium penetration — stopped by aluminum. emits a (anti)neutrino too Gamma (γ) A high-energy photon from a nucleus. A same, Z same No charge, no mass. MOST penetrating — needs thick lead. often follows α or β decay α (He nucleus), β (electron), γ (photon) — rising penetration, conserve A & Z. The Review Hub · AP Physics 2 · Unit 7
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How to use the visual review

Spend 30 seconds per slide before clicking next. Look at the diagram, then ask yourself: "Could I draw this from memory and explain it?"

Use the fullscreen button () on desktop for the best experience. Use arrow keys to navigate. Tap "Show all slides" to jump around.

This is great for review the night before the exam — fast, visual, and covers everything you need to remember about Unit 7's modern physics.