Amplitude is the maximum distance the wave moves away from its rest position. On a displacement graph it is measured from the middle line up to a crest or down to a trough, never from crest to trough.
This lesson belongs to wave behaviour. It follows naturally from connecting wavelength, frequency and speed, because the same graph can give you both amplitude and wavelength.
What does amplitude tell you?
The rest position, or equilibrium position, is where the medium sits when there is no wave. A wave displaces the medium above and below that line.
The size of the largest displacement is the amplitude. It describes how much energy the wave carries: a larger amplitude means a louder sound or a brighter light, for the same frequency.
Amplitude does not change the frequency, the wavelength or, in the same medium, the speed. Those belong to the source and the medium.
How do I read it from a graph?
- Find the rest line. On a displacement graph it is usually the horizontal axis. If the graph does not start from zero, use the line halfway between the crests and troughs.
- Read one crest and one trough against the vertical scale.
- Measure from the rest line, not between crest and trough.
- Check by halving: half of the crest-to-trough distance gives the same amplitude.
- Give the unit shown on the vertical axis, such as mm or cm.
Worked example
Invented data: a graph shows water level against distance for a ripple.
The vertical scale is height above the tank floor. Crests reach 12.0 cm and troughs fall to 8.0 cm. Find the amplitude.
Step 1, find the rest line: halfway between 12.0 and 8.0 is (12.0 + 8.0) ÷ 2 = 10.0 cm.
Step 2, measure from the rest line: crest 12.0 − 10.0 = 2.0 cm. Trough 10.0 − 8.0 = 2.0 cm.
Step 3, check: crest to trough is 4.0 cm, and half of that is 2.0 cm.
The amplitude is 2.0 cm. Reading 12.0 cm straight off the scale would have been wrong, because the axis does not start at the rest line.
The mistake to watch for
The two classic slips are reading crest to trough, or reading a crest height from the bottom of the scale.
Mistaken answer: amplitude = 4.0 cm (crest to trough)
The student measured the full height of the wave. Amplitude is half of that, 2.0 cm.
The correction is to ask “from where?” before reading any number. If the answer is not “from the rest position”, move the ruler or your eye until it is. The bounds and rounding explainer shows how much a reading like 12.0 cm can vary with the scale’s precision.
Check yourself
1. A displacement graph has crests at +2.5 cm and troughs at −2.5 cm. State the amplitude.
Show answer
The rest line is at 0. The amplitude is 2.5 cm.
2. On a graph the distance from crest to trough is 9.0 cm. What is the amplitude?
Show answer
Amplitude is half of crest to trough: 9.0 ÷ 2 = 4.5 cm.
3. A speaker plays the same note, then the volume is turned up. Which quantity changes, and which stay the same?
Show answer
The amplitude increases. The frequency (pitch), the wavelength and the speed of the sound stay the same.
Where this leads next
Amplitude is one feature of a wave. Next, see what happens to the wavefronts when a wave meets a boundary in reflection and refraction using wavefronts.
Students know the definition but can still lose the mark on a graph with an unusual scale. Our online one-to-one Physics tuition lets a teacher watch how you read graphs and correct that habit at once.