Measuring a wavelength

The idea

The wavelength λ is the distance between two consecutive crests or two consecutive troughs; its SI unit is the metre (m).

How long is one wave?

A sound wave drawn as a curve has a pattern that repeats along the distance axis.

Between which two points do you think the length of one wave is measured?

• Two consecutive crests
• A crest and the axis below it
• The first crest and the last trough

Crest to crest

Set the wavelength with the slider. Then choose whether to measure between crests or between troughs. The numbers are only examples.

One wave in length

The distance between two consecutive crests, or two consecutive troughs, is the wavelength. We write it with the Greek letter λ (lambda), and measure it in metres.

The wavelength λ is the distance between two consecutive crests or two consecutive troughs. Its SI unit is the metre (m).

Notes

The wavelength λ is the distance between two consecutive crests (or troughs). Its SI unit is the metre (m).

Check yourself

Which distance is the wavelength?

Two consecutive crests of a wave are at 3 m and 7 m along the distance axis (an example). What is the wavelength, in m?

Answer: 4 m

Wavelength = 7 m − 3 m = 4 m.

What is the SI unit of wavelength?

Which letter is usually used to represent wavelength?

• The distance between two consecutive crests — correct. Yes!
• The distance from a crest to the next trough. The wavelength is the distance between two consecutive crests or two consecutive troughs.
• The height of one crest above the dashed line. That is not the distance along the axis. Wavelength is the distance between two consecutive crests or two consecutive troughs.
• The hertz (Hz). The hertz is the unit of frequency. The SI unit of wavelength is the metre.
• The metre (m) — correct. Yes!
• The second (s). The second is the SI unit of time period. The SI unit of wavelength is the metre.
• ν (nu). The Greek letter nu represents frequency.
• T. The letter T represents the time period.
• λ (lambda) — correct. Yes!