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〜 Unit 6 · Waves, Sound, & Physical Optics Flashcards Cheat Sheet Essentials Visual Review MC Practice FRQ Practice

AP Physics 2 Unit 6 Visual Review

A topic-by-topic visual walkthrough of Unit 6: Waves, Sound & Physical Optics — wave properties, the Doppler effect, interference and standing waves, diffraction, and thin films.

← Back to Unit 6 hub
TOPIC 6.1 Properties of Wave Pulses & Waves amplitude A wavelength λ Waves transfer energy, not matter A pulse is a single disturbance; a wave is a repeating one. TRANSVERSE: medium moves ⊥ to travel (light). LONGITUDINAL: medium moves ∥ to travel (sound). Amplitude sets energy/intensity carried. Pulses reflect & can invert A pulse hitting a FIXED end reflects inverted; a FREE end reflects upright. Speed depends on the medium, not amplitude. 〰Waves carry energy, not matter; transverse ⊥, longitudinal ∥ to travel. The Review Hub · AP Physics 2 · Unit 6 TOPIC 6.2 Periodic Waves v = f λ f = 1 / T The quantities f = frequency (Hz), T = period (s). λ = wavelength (m), v = wave speed (m/s). Speed is set by the MEDIUM. Change f and λ adjusts so v stays constant. Higher f → shorter λ (in the same medium). Worked example A sound wave: f = 340 Hz, v = 340 m/s. λ = v/f = 340/340 = 1.0 m Its period: T = 1/f = 1/340 ≈ 2.9 ms Sound in air ≈ 343 m/s at room temperature. v = fλ and f = 1/T; wave speed is fixed by the medium. The Review Hub · AP Physics 2 · Unit 6 TOPIC 6.3 Boundary Behavior & Polarization At a boundary A wave meeting a new medium partly REFLECTS and partly TRANSMITS. Into a slower/denser medium: reflected pulse INVERTS (fixed-end behavior). Into a faster medium: reflected pulse stays UPRIGHT. Frequency is unchanged on transmission; speed & wavelength change together. Polarization Only TRANSVERSE waves can be polarized. filter only one orientation passes A polarizing filter blocks all but one orientation of the oscillation. Longitudinal waves (sound) CANNOT be polarized — a proof they're transverse for light. At boundaries waves reflect + transmit; only transverse waves polarize. The Review Hub · AP Physics 2 · Unit 6 TOPIC 6.4 Electromagnetic Waves Light is an EM wave c = f λ = 3.0×10⁸ m/s Oscillating E and B fields, perpendicular to each other and to travel direction. EM waves need NO medium — they travel through vacuum at speed c. the EM spectrum (low f → high f) radio IR visible UV X/γ All travel at c in vacuum; they differ only in frequency & wavelength. Higher f = higher photon energy (E = hf). Slowing in matter In a medium light slows to v = c/n; frequency stays the same, so wavelength shortens (λ = λ₀/n). EM waves are ⊥ E & B fields; c = fλ, no medium needed. The Review Hub · AP Physics 2 · Unit 6 TOPIC 6.5 The Doppler Effect lower f (behind) higher f (ahead) moving source bunches wavefronts ahead Motion shifts the frequency Relative motion between source and observer changes the OBSERVED frequency. Approaching → higher f (shorter λ). Receding → lower f (longer λ). The emitted frequency never actually changes. Everyday & cosmic examples A passing siren drops in pitch; light from receding galaxies is "redshifted." Motion shifts observed f: approaching = higher, receding = lower. The Review Hub · AP Physics 2 · Unit 6 TOPIC 6.6 Wave Interference & Standing Waves two waves superpose → standing wave nodes (no motion) Superposition principle Overlapping waves ADD. In phase → CONSTRUCTIVE (bigger). Out of phase → DESTRUCTIVE (cancel). Standing wave = two identical waves traveling opposite ways; NODES fixed, ANTINODES swing max. Resonance on strings & in pipes String / open pipe: λₙ = 2L/n. Pipe closed at one end: λₙ = 4L/n (odd n only). f = v/λ sets the harmonics. Waves superpose (add); standing waves have fixed nodes; λₙ = 2L/n. The Review Hub · AP Physics 2 · Unit 6 TOPIC 6.7 Diffraction waves spread out after a narrow gap Bending around obstacles & openings Waves spread when passing an edge or a gap — a hallmark of wave behavior. The effect is STRONGEST when the gap size is comparable to the wavelength λ. Small gap (≈ λ): lots of spreading. Gap ≫ λ: little spreading (nearly straight). Why you hear around corners but can't see around them — sound λ is much larger. Waves bend around gaps/edges; spreading is greatest when gap ≈ λ. The Review Hub · AP Physics 2 · Unit 6 TOPIC 6.8 Double-Slit Interference & Diffraction Gratings Bright-fringe condition d sin θ = m λ d = slit spacing, m = 0, 1, 2, … the order of the bright fringe (maximum). Dark fringes: d sin θ = (m + ½)λ. On a screen: bright spot spacing Δy ≈ λL/d. alternating bright & dark fringes on screen Gratings sharpen the pattern A grating has many slits → same d sin θ = mλ but far narrower, brighter maxima. It splits white light into a spectrum. Bright fringes at d sin θ = mλ; a grating makes them sharper & brighter. The Review Hub · AP Physics 2 · Unit 6 TOPIC 6.9 Thin-Film Interference thin film (thickness t) top & bottom reflections interfere Why soap bubbles & oil shimmer Light reflects off both the top and bottom of a thin film; the two reflected waves interfere. The extra path ≈ 2t sets which colors add (bright) or cancel (dark). A ½-λ PHASE FLIP happens on reflecting off a higher-n medium — it flips the bright/dark condition. Used in anti-reflective lens coatings. Top & bottom reflections interfere via path ≈ 2t; mind the ½-λ phase flip. The Review Hub · AP Physics 2 · Unit 6
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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 6's waves, sound, and physical optics.