Reading wave graphs

What you will learn

Spatial and temporal representations; amplitude, wavelength and period from graphs.

Specification mapping: A1.1.2

Prerequisites: A1.1.1

Physical principles

A displacement–distance graph is a snapshot at one time. Its horizontal axis is position in metres and the separation of adjacent crests is wavelength. A displacement–time graph follows one location as time passes. Its horizontal axis is time in seconds and the separation of adjacent crests is period.

Check both axis labels before extracting a number. Identical-looking curves can represent different measurements. A crest and the next zero crossing are separated by a quarter cycle, not a full cycle. Points with equal displacement need not have the same phase: one may be moving upwards while the other is moving downwards.

Use the zero-displacement line to find amplitude, including when a graph has a non-zero plotting baseline. Read several complete repeats and divide by their number when this improves precision. The slope of a displacement–time curve describes particle velocity; it is not the propagation speed of the wave.

Scientific diagram and interpretation

Worked examples and applications

Common misconceptions

A time axis does not give wavelength. Equal displacements do not establish equal phase.

Examination technique

Annotate the interval between equivalent points and show subtraction of axis readings.

For an explanation, connect the physical cause to its effect. For a calculation, state the model, rearrange before substitution, retain intermediate digits, and finish with units. For a data question, refer to the actual readings and distinguish a trend from proof of a cause.

Check your understanding

Five questions. Choose an option, then Check Answer. Read the feedback and use Try Again to reset. Use Tab between controls, arrow keys within radio groups, and Enter or Space on buttons. These local practice answers are not saved.