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

Unit 14: Waves, Sound, and Physical Optics

Unit 14 treats light and sound as waves. Wave properties, superposition, standing waves, and the Doppler effect lead into the interference and diffraction phenomena that geometric optics cannot explain. Double-slit and single-slit patterns are the standard quantitative applications.

Position: 6 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.

Wave properties and models: Transverse and longitudinal waves, and the relationship among wavelength, frequency, and wave speed.
Superposition and interference: Constructive and destructive interference, path-length difference, and phase relationships.
Standing waves: Nodes and antinodes, harmonics on strings and in open and closed pipes, and resonance conditions.
Sound and the Doppler effect: Sound as a pressure wave, intensity, and the frequency shift produced by relative motion.
Diffraction and interference of light: Double-slit and single-slit patterns, diffraction gratings, and locating bright and dark fringes.
Polarization: What polarization reveals about the transverse nature of light, and the effect of successive filters.

Study tips

How to practice Waves and Physical Optics.

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

Sketch the standing-wave harmonics for a string and for both pipe types and derive the wavelength each time.
Reduce interference problems to a path-difference comparison with the wavelength before using any formula.
Reason the Doppler shift direction from the physical picture rather than memorizing the sign in the equation.

Where this fits

Unit 14: Waves, Sound, and Physical Optics is 6 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 Waves and Physical Optics 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
A sound wave of frequency 170 Hz travels through air at 340 m/s. What is the wavelength of the wave?
1. A0.50 m
2. B2.0 m
3. C4.0 m
4. D5.8 x 10^4 m
Show answer
Answer: B. 2.0 m
The wave relationship v = f(lambda) gives lambda = 340/170 = 2.0 m. The 5.8 x 10^4 m choice multiplies speed by frequency instead of dividing, and 0.50 m inverts the ratio.
Key insight: In a given medium the wave speed is fixed, so higher frequency always means shorter wavelength.
Question 2Foundational
A string of length 1.2 m is fixed at both ends and vibrates in its fundamental standing-wave pattern. What is the wavelength of the wave on the string?
1. A0.60 m
2. B1.2 m
3. C2.4 m
4. D4.8 m
Show answer
Answer: C. 2.4 m
The fundamental has a node at each fixed end and a single antinode in the middle, so the string holds half a wavelength and lambda = 2L = 2.4 m. The 1.2 m choice sets the wavelength equal to the string length, which is the second harmonic instead.
Key insight: Fixed ends must be nodes, so the allowed wavelengths on a string are 2L/n for integer n.
Question 3Intermediate
Light of wavelength 600 nm passes through two narrow slits separated by 2.0 x 10^-5 m and forms a pattern on a screen 2.0 m away. How far from the central bright fringe is the first-order bright fringe?
1. A0.60 cm
2. B3.0 cm
3. C6.0 cm
4. D12 cm
Show answer
Answer: C. 6.0 cm
For small angles the bright fringes lie at y = m(lambda)L/d, so y = (600 x 10^-9)(2.0)/(2.0 x 10^-5) = 0.060 m = 6.0 cm. The 12 cm choice uses m = 2, which would locate the second-order fringe.
Key insight: Fringe spacing grows when the wavelength or screen distance grows and shrinks when the slits move farther apart.
Question 4Advanced
A car sounding a horn of constant frequency drives directly toward a stationary listener at a constant speed on a straight road. What does the listener hear while the car is approaching?
1. AA constant frequency higher than the emitted frequency
2. BA constant frequency lower than the emitted frequency
3. CA frequency that rises steadily as the car gets closer
4. DA frequency equal to the emitted frequency
Show answer
Answer: A. A constant frequency higher than the emitted frequency
The Doppler shift depends on the relative speed along the line between source and observer, not on the separation, so a car approaching at constant speed produces a steady, higher pitch. The pitch drops abruptly only after the car passes and begins to recede.
Key insight: Approaching sources compress the wavefronts by a fixed factor set by their speed, so the pitch is raised but steady.
Question 5Intermediate
Unpolarized light of intensity I0 passes through a single ideal polarizing filter. What is the intensity of the light that emerges?
1. AZero
2. BOne quarter of I0
3. CHalf of I0
4. DAll of I0
Show answer
Answer: C. Half of I0
Unpolarized light contains all polarization directions equally, so an ideal filter transmits the average of cos^2, which is one half. A second filter would then follow Malus's law, but the first one always cuts unpolarized light in half regardless of its orientation.
Key insight: The first polarizer halves unpolarized light; every polarizer after that depends on the angle between the transmission axes.

Continue through AP Physics 2: Algebra-Based

Practice Waves and Physical Optics deliberately.

Create your Arcade Academy workspace to turn this topic list into spaced-repetition reviews, linked notes, and progress you can act on.

Create your account

Unit 14: Waves, Sound, and Physical Optics