Transverse and longitudinal waves

AS Level Physics · Waves. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

In a transverse wave the particles vibrate at right angles to the direction in which the energy travels. Waves on a rope, water ripples and all electromagnetic waves are transverse. In a longitudinal wave the particles vibrate along the direction of energy travel, making compressions and rarefactions. Sound is longitudinal.

Both types carry energy, obey v = fλ and can be reflected, refracted and diffracted. Only transverse waves can be polarised. Sound needs a medium because it needs particles to vibrate.

Two graphs describe a wave. Displacement against distance is a picture at one instant and gives the wavelength. Displacement against time follows one point and gives the period. Both give the amplitude.

Rules to remember

Common mistake

Students read the wavelength from a displacement–time graph. That graph gives the period. Always check the label on the horizontal axis before you read anything.

Worked example

A sound wave of frequency 250 Hz travels through air at 340 m s−1. What is the distance between the centre of a compression and the centre of the nearest rarefaction?

  1. Wavelength λ = v ÷ f = 340 ÷ 250 = 1.36 m
  2. Neighbouring compressions are one wavelength apart, and a rarefaction lies halfway between them
  3. Distance = λ ÷ 2 = 1.36 ÷ 2 = 0.68 m

Answer: 0.68 m

Practice questions

Try each one, then open the answer.

1. Which statement is correct for a longitudinal wave?

  1. A
    the oscillations are perpendicular to the direction of energy transfer
  2. B
    the oscillations are parallel to the direction of energy transfer
  3. C
    it cannot travel through a gas
  4. D
    it can be polarised
Show answer

Answer: B. In a longitudinal wave (sound) particles oscillate along the direction of travel, forming compressions and rarefactions. Only transverse waves can be polarised.

2. Why can sound not travel through a vacuum, although light can?

  1. A
    sound is a transverse wave
  2. B
    light is a longitudinal wave
  3. C
    sound is a mechanical wave and needs particles of a medium to oscillate
  4. D
    sound travels faster than light
Show answer

Answer: C. Sound is passed on by particles of a medium pushing on their neighbours. Light is an electromagnetic wave and needs no medium.

3. Which property is shared by transverse and longitudinal waves?

  1. A
    both can be polarised
  2. B
    both need a medium to travel through
  3. C
    both involve oscillations perpendicular to the direction of travel
  4. D
    both transfer energy without any net transfer of matter
Show answer

Answer: D. All progressive waves transfer energy while the particles only oscillate about fixed positions. Only transverse waves can be polarised, and electromagnetic waves need no medium.

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