Amplitude is the quiet detail that decides whether a wave problem feels obvious or unfair. You look at a neat sine curve, you label a few points, and then the question asks about energy, intensity, or a “maximum displacement” buried in text. In IB Physics, that one word, amplitude, often separates “I know this” from “I lost three marks for something basic.”
This guide breaks amplitude down the way examiners actually use it: as a definition, a graph-reading skill, and a gateway to energy reasoning.
Quick amplitude checklist (save this for IB Physics)
Before you calculate anything in IB Physics, run this quick check:
-
Amplitude is measured from the equilibrium (midline) to a crest or trough.
-
Crest-to-trough height is 2A, not A.
-
Amplitude affects energy and intensity, not wave speed.
-
On graphs, amplitude is the maximum vertical displacement.
-
In longitudinal waves (sound), amplitude means maximum pressure/density variation from equilibrium.
If you want the syllabus-aligned home base for waves, start with Oscillations and Waves (IB Physics).
What amplitude actually means in IB Physics
In IB Physics, amplitude (usually written as (A)) is the maximum displacement of a point on a wave from its equilibrium position.
That definition is short, but it carries two important exam clues:
-
Maximum: not an average, not a typical value, not “most of the time.” The biggest displacement.
-
From equilibrium: the reference line matters. If you measure from crest to trough, you doubled it.
Amplitude in transverse vs longitudinal waves
-
Transverse waves (string waves, water surface waves, electromagnetic waves): amplitude is the vertical displacement from the center line to a crest (or to a trough).
-
Longitudinal waves (sound in air): particles oscillate parallel to motion, so amplitude is tied to the maximum compression or rarefaction relative to the equilibrium pressure/density.
For wave characteristics in one place (including wavelength, frequency, and the standard wave equation), use Wave Characteristics Notes (4.3).
Why amplitude matters: energy scales with A²
Here’s the part students remember vaguely, but exam questions punish vaguely.
For many wave systems, the energy transported (or intensity) is proportional to the square of the amplitude:
So in IB Physics terms:
-
If amplitude doubles, energy becomes four times larger.
-
Small amplitude errors become big energy errors.
That’s why questions that look “graphy” suddenly turn into “explain loudness” or “compare intensities.” Louder sound corresponds to larger pressure amplitude; brighter light corresponds to larger electric field amplitude.
To train this under exam pressure, build short sets from the Topic C: Wave behaviour Questionbank and review mistakes using RevisionDojo’s Grading tools and AI Chat to understand what the markscheme expected.
How to find amplitude on graphs (the IB Physics way)
Amplitude shows up most often on two graph types.
Displacement-time graphs
A displacement-time graph tracks one point in the medium as time passes. Amplitude is the maximum vertical displacement above (or below) the equilibrium line.
Displacement-position graphs
A displacement-position graph is a snapshot in space at one instant. Again, amplitude is the maximum vertical distance from the equilibrium line.
The skill isn’t “spot the tallest crest.” The skill is: identify the equilibrium line first, then measure to a peak.
If you want this to stop being theoretical, combine C.2 Wave Model Notes with quick retrieval practice using RevisionDojo Flashcards.
Common amplitude mistakes (and how IB Physics marks them)
Amplitude errors are predictable, which is good news: you can pre-fix them.
-
Mistake: Measuring crest-to-trough as amplitude. That’s (2A). Many questions quietly test this.
-
Mistake: Mixing up amplitude with wavelength. Wavelength is horizontal spacing; amplitude is vertical displacement.
-
Mistake: Thinking amplitude changes speed. In basic models, wave speed depends on the medium (tension, density, refractive index), not amplitude.
-
Mistake: Ignoring damping. Real systems lose energy, so amplitude often decreases with time.
If you’re studying fast, the routines in How to Cram IB Physics in 1 Week pair well with amplitude practice: Notes, Questions, Review mistakes, Flashcards.
How amplitude connects to standing waves and interference
Amplitude becomes more interesting when waves overlap.
-
In interference, amplitudes add (constructive) or subtract (destructive). See Interference Notes (9.3).
-
In standing waves, amplitude depends on position: nodes have zero amplitude; antinodes have maximum amplitude. Use Standing Waves Notes (4.5) and Formation and properties of standing waves (C.4.1).
This is also where RevisionDojo’s Mock Exams and Predicted Papers help: amplitude questions get tougher when they’re hidden inside standing wave contexts.
Bringing it home: make amplitude easy marks in IB Physics
Amplitude is one of those IB Physics ideas that feels too simple to revise, until it costs you marks. Treat it like a routine: find the equilibrium line, measure to the maximum displacement, and remember that energy scales with (A^2).
When you want amplitude to stop being a definition and start being a reflex, use RevisionDojo as your loop: study the Wave Characteristics Notes (4.3), drill with the Questionbank, lock it in with Flashcards, and check your thinking with AI Chat. That’s how small fundamentals become reliable exam points in IB Physics.
