What happens when a wave meets a boundary, and what does polarization reveal about light?
Topic 14.3 Boundary Behavior of Waves and Polarization: describe reflection and transmission of waves at boundaries and the polarization of transverse waves.
A focused answer to AP Physics 2 Topic 14.3, covering what happens when a wave meets a boundary (reflection, transmission and inversion), the constancy of frequency across a boundary, and the polarization of transverse waves as evidence that light is transverse, with full worked examples.
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What this topic is asking
The College Board (Topic 14.3) wants you to describe what happens when a wave meets a boundary, reflection, transmission and possible inversion, and to explain the polarization of transverse waves as evidence that light is transverse.
Waves at a boundary
At any boundary, some energy bounces back and some passes through, which is why you both hear an echo and lose sound through a wall. The key conserved quantity is frequency: the source sets it, so the transmitted wave oscillates at the same rate even though it travels at a new speed with a new wavelength. The inversion rule is tested directly: a pulse on a string hitting a fixed end (or a heavier string) flips over on reflection, while one hitting a free end (or a lighter string) reflects upright.
Polarization and the transverse nature of light
Polarization is possible only because a transverse wave can oscillate in different directions perpendicular to its travel; a polariser then selects one of those directions. A second polariser turned degrees to the first (crossed polarisers) blocks the light entirely, since the first has already removed everything along the second's blocked axis. The decisive physics point is what this reveals: light can be polarized, so light is transverse, while sound cannot be polarized, so sound is longitudinal (its oscillation has only one direction, along travel). Polarizing sunglasses and LCD screens exploit this. The strategic role of this topic is that boundary behavior sets up reflection, echoes and the standing waves of Topic 14.6 (which form when reflected waves overlap the incident ones), while polarization cements the distinction between transverse and longitudinal waves introduced in Topic 14.1 and identifies light, the subject of Topic 14.4, as transverse.
Try this
Q1. State what happens to a wave's frequency when it crosses into a new medium. [1 point]
- Cue. It stays the same (frequency is set by the source); the speed and wavelength change.
Q2. State what the ability to polarize light shows about the nature of light. [1 point]
- Cue. That light is a transverse wave (only transverse waves can be polarized).
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2024 (style)6 marksSection II (short FRQ). A wave pulse travels along a light string toward a junction with a heavier string. (a) Describe what happens to the pulse at the boundary in terms of reflection and transmission. (b) State whether the reflected pulse is inverted, and justify it. (c) State what happens to the frequency of the transmitted wave compared with the incident wave.Show worked answer →
A 6-point FRQ on boundary behavior.
(a) Reflection and transmission (2 points): at the boundary, part of the pulse is reflected back along the light string and part is transmitted into the heavier string (at a different speed).
(b) Inversion (2 points): reflecting off a denser (heavier, slower) medium inverts the pulse (it flips upside down), like reflecting off a fixed end. Reflecting off a lighter medium would not invert it.
(c) Frequency (2 points): the frequency of the transmitted wave is unchanged; frequency is set by the source and is conserved across the boundary, while the speed and wavelength change.
Markers reward identifying partial reflection and transmission, inversion off a denser medium, and the unchanged frequency.
AP 2023 (style)1 marksSection I (multiple choice). Light can be polarized, but sound cannot. What does this demonstrate? (A) light travels faster than sound (B) light is a transverse wave and sound is longitudinal (C) sound has no frequency (D) light has no amplitude. Justify your reasoning.Show worked answer →
A 1-point MCQ on polarization. The answer is (B).
Only transverse waves can be polarized, because polarization restricts the oscillation to one plane perpendicular to the travel direction. Light is transverse, so it can be polarized; sound is longitudinal (oscillating along the travel direction), so it cannot. The trap is (A): speed is irrelevant to whether a wave can be polarized.
Related dot points
- Topic 14.1 Properties of Wave Pulses and Periodic Waves: describe transverse and longitudinal waves and apply v = f lambda to periodic waves.
A focused answer to AP Physics 2 Topics 14.1 and 14.2, covering wave pulses and periodic waves, the distinction between transverse and longitudinal waves, the meaning of amplitude, wavelength, frequency and period, the wave equation v = f lambda, and the fact that a medium does not travel with the wave, with full worked examples.
- Topic 14.4 Electromagnetic Waves: describe electromagnetic waves, their speed in vacuum, and the electromagnetic spectrum.
A focused answer to AP Physics 2 Topic 14.4, covering electromagnetic waves as oscillating electric and magnetic fields, their constant speed in vacuum, the wave equation c = f lambda for light, the organization of the electromagnetic spectrum by frequency and wavelength, and the transverse nature of light, with full worked examples.
- Topic 14.6 Wave Interference and Standing Waves: apply superposition to interference and find the harmonics of standing waves.
A focused answer to AP Physics 2 Topic 14.6, covering the superposition principle, constructive and destructive interference, the formation of standing waves with nodes and antinodes, the harmonics of a string and a pipe, and resonance, with full worked examples.
- Topic 13.1 Reflection: apply the law of reflection and the ray model of light to plane surfaces.
A focused answer to AP Physics 2 Topic 13.1, covering the ray model of light, the law of reflection that the angle of incidence equals the angle of reflection, the distinction between specular and diffuse reflection, and image formation in a plane mirror, with full worked examples.
- Topic 14.5 The Doppler Effect: explain the shift in observed frequency when a wave source and observer move relative to each other.
A focused answer to AP Physics 2 Topic 14.5, covering the Doppler effect for sound and light, the rise in observed frequency on approach and the fall on recession, the physical reason in terms of bunched and stretched wavefronts, and the redshift of receding light, with full worked examples.
Sources & how we know this
- AP Physics 2: Algebra-Based Course and Exam Description — College Board (2024)