---
title: "Phase Shift — AP Physics 2 Definition & Exam Guide"
description: "A phase shift is a 180° flip a wave can pick up when it reflects off a slower medium. Learn the speed rule, the string trick, and how AP Physics 2 tests it."
canonical: "https://fiveable.me/ap-physics-2-revised/key-terms/phase-shift"
type: "key-term"
subject: "AP Physics 2"
unit: "Unit 14"
---

# Phase Shift — AP Physics 2 Definition & Exam Guide

## Definition

In AP Physics 2, a phase shift is the 180° (half-wavelength) inversion a reflected wave picks up at a boundary when the transmitted wave enters a medium where the wave speed decreases; if the speed increases, the reflected wave comes back upright with no phase shift.

## What It Is

A phase shift is a change in where a [wave](/ap-physics-2-revised/unit-14/1-properties-of-wave-pulses-and-waves/study-guide/Ql0FLnrI6dIHcNlL "fv-autolink") is in its cycle. On the [AP Physics 2 exam](/ap-physics-2-revised/ap-physics-2-exam "fv-autolink"), the phase shift you care about happens at a boundary between two media. When a wave hits a boundary, part of it reflects and part of it transmits. The question is whether the reflected piece comes back flipped upside down (a 180° phase shift) or right-side up (no phase shift).

The CED gives you one clean rule. If the transmitted wave moves into a medium where the wave travels **slower**, the reflected wave is **inverted**. If the transmitted wave moves into a medium where the wave travels **faster**, the reflected wave is **not inverted**. Picture a pulse on a light string tied to a heavy rope. The wave slows down in the heavy rope, so the reflected pulse flips. The extreme case is a fixed end (or an electromagnetic wave hitting a perfect conductor), where essentially nothing transmits and the reflection always inverts. One more thing that never changes at a boundary, no matter what, is the [frequency](/ap-physics-2-revised/key-terms/frequency "fv-autolink").

## Why It Matters

Phase shift lives in **[Topic 14.3](/ap-physics-2-revised/unit-14/3-boundary-behavior-of-waves-and-polarization/study-guide/KovuXqE175SrnAJs "fv-autolink"), Boundary Behavior of Waves and Polarization**, in **[Unit 14](/ap-physics-2-revised/unit-14 "fv-autolink"): Waves, Sound, and Physical Optics**. It directly supports learning objective **14.3.A**, describing the interaction between a wave and a boundary. The essential knowledge spells out exactly the inversion rule above, plus the fact that frequency stays constant across a boundary. This matters beyond one topic because phase is the currency of interference. Whether two waves add constructively or destructively depends on their phase relationship, and a reflection phase shift can flip that relationship. That extra 180° from a reflection is the twist that makes thin-film interference problems tricky, and it explains why standing waves on a string need a node at a fixed end.

## Connections

### Wave speed and string density (Unit 14)

[Wave speed](/ap-physics-2-revised/key-terms/wave-speed "fv-autolink") on a string depends on tension and linear density, and heavier strings carry slower waves. That's why exam questions hand you μ₂ > μ₁ instead of speeds. They expect you to translate 'denser string' into 'slower wave' and then apply the inversion rule.

### Interference and path difference (Unit 14)

Interference outcomes depend on total phase difference, which comes from two sources. [Path length difference](/ap-physics-2-revised/key-terms/path-length-difference "fv-autolink") is one, and reflection phase shifts are the other. A single 180° reflection shift swaps every constructive condition for a destructive one, which is the whole game in thin-film problems.

### Standing waves and fixed ends (Unit 14)

A fixed end is the ultimate slow medium, so reflections there always invert. That forced inversion is what creates a [node](/ap-physics-2-revised/unit-14/6-wave-interference-and-standing-waves/study-guide/3twkmfrYKPDOuep9 "fv-autolink") at a fixed boundary, which sets up the harmonic patterns for strings and pipes.

### [Intensity (Unit 14)](/ap-physics-2-revised/key-terms/intensity)

Phase shift tells you whether waves are flipped; intensity tells you how much energy they carry. When a wave splits at a boundary, the energy divides between the reflected and transmitted waves, but the phase rule only controls the orientation of the reflected one.

## On the AP Exam

This shows up most often as a multiple-choice stem that gives you two media and asks about the reflected wave. Sometimes you get speeds directly, like a wave going from 300 m/s to 150 m/s (slower, so the reflection is inverted, a 180° shift). Sometimes you get string densities, like μ₂ > μ₁ (denser means slower, so inverted again). Sound versions work the same way, like a sound wave going from water at 1500 m/s into a liquid at 1200 m/s, where the reflected wave ends up 180° out of phase with the incident wave. You may also see an electromagnetic wave hitting a perfect conductor, which reflects fully inverted. Your job in every version is the same two-step move. First figure out whether the wave speeds up or slows down in the second medium, then apply the rule. And remember the trap answer that frequency changes at the boundary. It never does.

## phase shift vs phase difference from path length

A reflection phase shift and a path-length phase difference both change interference, but they come from different places. A reflection phase shift is an instant 180° flip that happens at a boundary when the wave reflects off a slower medium. A path-length phase difference builds up gradually because one wave travels farther than another. In interference problems you have to add both effects together. Forgetting the reflection shift is the classic way to get constructive and destructive conditions backwards.

## Key Takeaways

- A reflected wave is inverted (180° phase shift) when the transmitted wave enters a medium where the wave speed decreases.
- A reflected wave is not inverted when the transmitted wave enters a medium where the wave speed increases.
- On strings, denser means slower, so a pulse reflecting off a heavier string comes back flipped.
- Frequency never changes when a wave crosses a boundary; only speed and wavelength change.
- A fixed end, or an EM wave hitting a perfect conductor, always produces a fully inverted reflection.
- In interference problems, count both reflection phase shifts and path-length differences before deciding constructive or destructive.

## FAQs

### What is a phase shift in AP Physics 2?

It's the change in a wave's phase at a boundary, most importantly the 180° inversion a reflected wave gets when the transmitted wave moves into a slower medium. It's tested under learning objective 14.3.A in Unit 14.

### Does a wave always flip when it reflects?

No. It only inverts if the second medium carries the wave more slowly (like a light string attached to a heavy rope, or a fixed end). If the second medium is faster, the reflection comes back upright.

### Does the frequency change when a wave hits a boundary?

No, never. Frequency is set by the source and stays the same across the boundary. The speed and wavelength change instead, which is a favorite distractor on multiple-choice questions.

### How is a reflection phase shift different from a path difference?

A reflection phase shift is a one-time 180° flip that happens at the boundary itself. A path difference is a phase gap that grows because one wave travels a longer distance. Interference questions often require adding both together.

### What happens when an electromagnetic wave hits a perfect conductor?

The wave is fully reflected and inverted, a 180° phase shift. It behaves like the fixed-end case for a wave on a string, since essentially nothing transmits through the conductor.

## Related Study Guides

- [14.3 Boundary Behavior of Waves and Polarization](/ap-physics-2-revised/unit-14/3-boundary-behavior-of-waves-and-polarization/study-guide/KovuXqE175SrnAJs)

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