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What is a wave?

Waves, sound and light · level 5, lesson 1 of 6

A question to start

A duck bobs up and down on a pond as ripples pass, but it does not move along with them. What is a wave carrying, if not the water?

The idea

A wave carries energy from one place to another without carrying the stuff it travels through. Each bit of water, air or rope only moves back and forth around where it started.

There are two kinds:

  • Transverse waves: the vibration is at right angles to the way the wave travels. A wave on a rope, and light.
  • Longitudinal waves: the vibration is along the way the wave travels. Sound.
wavelength λcresttroughamplitude

The parts of a wave:

  • Amplitude: the biggest distance from the middle line. It sets how loud or bright the wave is.
  • Wavelength \(\lambda\) (lambda): the distance from one crest to the next.
  • Frequency \(f\): how many waves pass each second, in hertz.
  • Period \(T\): the time for one wave, \(T = 1/f\).

Worked examples

1. The duck. It moves up and down as each crest and trough passes, but stays in the same place: the energy moves on, the water does not.

2. A period. A wave with \(f = 50\) Hz has \(T = 1/50 = 0.02\) s.

3. Counting waves. 12 crests pass a post in 4 seconds: \(f = 12/4 = 3\) Hz.

Try it

  1. What does a wave carry?
  2. Is sound a transverse or a longitudinal wave?
  3. Find the period of a 200 Hz sound.
  4. A wave's period is 0.5 s. What is its frequency?
Answers
  1. Energy.
  2. Longitudinal.
  3. \(1/200 = 0.005\) s.
  4. \(1/0.5 = 2\) Hz.

Common mistake

Measuring amplitude from crest to trough. The amplitude is from the middle line to a crest: half the crest-to-trough height.

Where it leads