What it covers: Wave behavior in mechanical systems and light — mechanical wave properties, standing waves, sound and the Doppler effect, and the wave-based optics of interference and diffraction.
Exam weight: About 12–15% of the AP Physics 2 exam.
The big question: What behaviors are universal to all waves — mechanical or light — and how does interference reveal that light itself is a wave?
In the College Board CED: Unit 14: Waves, Sound, and Physical Optics (Topics 14.1–14.9) — the revised CED numbers Physics 2 units 9–15.
Key topics at a glance
Wave Properties
v = fλ. Wavelength, frequency, period (T = 1/f), and amplitude describe every wave, mechanical or electromagnetic.
Formed by two opposite-traveling waves superimposing. Nodes = zero displacement, antinodes = max displacement. Resonance amplifies natural frequencies.
Diffraction — bending of waves around obstacles or through openings.
Young's double-slit experiment — the classic experiment demonstrating light's wave nature via interference fringes.
Key themes to remember
Waves transfer energy, not matter. Individual particles oscillate in place; the disturbance (and its energy) is what propagates.
Standing waves and interference patterns are both consequences of superposition. Same underlying principle, different geometric setup (reflected wave vs. two coherent sources).
The Doppler effect is about relative motion, not the wave's intrinsic speed changing. The wave still travels at its normal speed through the medium — only the perceived frequency shifts.
Diffraction and interference are the smoking-gun evidence that light is a wave. Geometric optics (Unit 5) explains most everyday observations, but only wave behavior explains fringe patterns.
Resonance is how small driving forces produce large responses. Matching a system's natural frequency unlocks much greater amplitude than any other driving frequency.
Common exam traps
Don't confuse wave speed with particle speed. Wave speed (v = fλ) describes how fast the pattern propagates; individual particles oscillate at a much smaller speed around their equilibrium position.
The Doppler effect shifts frequency, not wave speed. The wave still travels at the same speed through the medium — what changes is how often crests arrive at the observer.
Nodes and antinodes alternate, evenly spaced. Don't assume node spacing is the full wavelength — adjacent nodes are λ/2 apart.
Bright fringes correspond to whole-wavelength path differences, not half. Mixing up the mλ and (m+½)λ conditions for bright vs. dark fringes is one of the most common errors.
Smaller slit spacing means WIDER fringe spacing — an inverse relationship that's easy to get backwards under exam pressure.
Resonance requires matching the driving frequency to a natural frequency — not just "shaking something hard." Amplitude only spikes dramatically at specific frequencies.