Amplitude, period, wave velocity, wavefront

Read this wave like a ruler and a stopwatch

time (s): 5. displacement (cm): 3

Drag the dot to the highest point and read its height. Then slide it to the next peak and see how much time has passed.

Predict, then check

Same graph skills, trickier numbers.

On a displacement–time graph, a student finds a peak at 0.5 s and the very next trough at 1.5 s. They say "the period is 1 s". What have they actually measured?

Waves · Wavefronts

Step through the views, then tap a part to read about it.

Neighbouring crest wavefronts are one wavelength apart.

Step through the views, then tap a part of the diagram.

Waves · Four key terms

Which term is it?

Pick a clue, then pick the term it belongs to.

8 of 8 still to sort.

Four words, four different jobs. Sort each clue under the term it describes.

What you need to know

• Start every reading at the rest line: the undisturbed position, with no wave going through.
• Amplitude is the maximum displacement from the rest position, up to a crest or down to a trough.
• Have a goDev says only crests count, so a wave with a crest 2 cm above the rest line beats one with a trough 4 cm below it. Is he right? Which has the bigger amplitude?

  No. The 4 cm trough wave has the bigger amplitude: 4 cm.

  Amplitude is the greatest distance from the rest position, and a trough counts just as much as a crest. Dev only checked the crests.

• Period is the time for one complete oscillation, measured in seconds (s).
• On a displacement–time graph, find it as the time between two matching points one full cycle apart.
• Have a goA graph has one peak at 0.5 s and the next peak at 2.5 s. What is the period?

  2 s

  The period is the gap between two matching points: 2.5 − 0.5 = 2 s. The 2.5 s is where the second peak sits, not how long a cycle takes.

• Wave velocity is how far the wave travels each second, measured in metres per second (m/s).
• A wavefront is a line joining points at the same point in their cycle, such as a line along one crest.
• Wavefronts are drawn at right angles to the direction the wave and its energy travel.
• Have a goA wave travels left to right across the page. Do its wavefronts run left to right, or up and down the page?

  Up and down the page, at right angles to the direction of travel.

  Wavefronts join points at the same point in their cycle and are drawn at right angles to the direction of travel, not along it.

• Two neighbouring crest wavefronts are one wavelength apart.

The big picture

Amplitude is how far a wave reaches from its rest line. Period is the time for one complete cycle. Wave velocity is how far the wave travels each second. A wavefront is a line along points at the same stage of the wave, at right angles to the direction of travel.

⚠ Watch out

Measuring amplitude from crest to trough. That span is two amplitudes, so it doubles the answer. Start at the rest line and stop at one crest or one trough.

Memory hook

Amplitude is a ruler from the rest line to one peak. Period is a stopwatch for one full cycle.

Check yourself

A crest is 5 cm above the rest line, with peaks at 2 s and 7 s. Find the amplitude and period. (Answer: 5 cm and 5 s.)