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12.1 The Nature of Waves

GCSE Physics · AQA · Chapter 12: Wave Properties

A free GCSE Physics lesson for AQA (8463) from OBA School: what waves transfer, mechanical and electromagnetic waves, transverse and longitudinal waves. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • State that waves transfer energy and information from place to place without transferring matter
  • Explain the difference between mechanical waves and electromagnetic waves
  • Describe a transverse wave and give examples
  • Describe a longitudinal wave, including compressions and rarefactions, and give examples
  • Describe evidence that it is the wave, not the material, that travels

Key words

  • Wave: A disturbance that travels from place to place, carrying energy (and often information) but not matter.
  • Oscillation: A repeated movement backwards and forwards (or up and down) about a fixed position. Also called a vibration.
  • Medium: The substance a wave travels through, such as water, air or rock.
  • Mechanical wave: A wave that is a vibration of a medium. It cannot travel through a vacuum. Examples: sound, water waves, seismic waves.
  • EM wave: An electromagnetic wave, such as light, radio or microwaves that can travel through a vacuum, at 3.0 × 10⁸ m/s.
  • Transverse wave: A wave whose oscillations are perpendicular (at 90°) to the direction of energy transfer.
  • Longitudinal wave: A wave whose oscillations are parallel to the direction of energy transfer.
  • Compression / rarefaction: In a longitudinal wave: a region where the particles (or coils) are squashed together / spread further apart.

What do waves do?

A wave is a way of moving energy from one place to another. Sunlight warms a car parked outside; ocean waves can move huge boulders on a beach; the sound of a car horn makes your ear drum move. In every case energy arrives, but the material between the source and you has not travelled to you.

Waves also carry information. A text message travels as radio waves to a phone mast, a fibre-optic cable carries internet data as pulses of light, and your friend’s voice reaches you as sound.

  • What it is; Mechanical waves a vibration of a substance (the medium); Electromagnetic waves a vibration of electric and magnetic fields
  • Needs a medium?; Mechanical waves yes – cannot cross a vacuum; Electromagnetic waves no – can cross empty space
  • Examples; Mechanical waves sound, water waves, waves on a rope or spring, seismic waves; Electromagnetic waves radio, microwaves, infrared, light, ultraviolet, X-rays, gamma rays
  • Speed; Mechanical waves depends on the medium (sound in air about 340 m/s); Electromagnetic waves all travel at 3.0 × 10⁸ m/s in a vacuum

Key fact: Waves transfer energy and information. They do not transfer matter.

Transverse waves

Tie one end of a long rope to a post and move the other end up and down. A wave travels along the rope, but each small piece of rope just moves up and down about its rest position. The oscillations are at right angles to the direction the wave carries energy. This is a transverse wave.

  • ripples and waves on the surface of water
  • waves on a stretched rope, string or guitar string
  • all electromagnetic waves (light, radio, X-rays …)
  • S-waves (secondary seismic waves) from earthquakes

Exam tip: Write “the oscillations are perpendicular to the direction of energy transfer”. Just saying “at right angles to the wave” does not always get the mark.

Longitudinal waves

Lay a slinky spring on a bench and push and pull one end repeatedly along its length. Pulses of squashed-up coils (compressions) and stretched-out coils (rarefactions) travel along the slinky. Each coil moves backwards and forwards along the line the energy travels. This is a longitudinal wave.

Sound is the most important longitudinal wave. A loudspeaker cone pushes and pulls on the air in front of it, making compressions (air at higher pressure) and rarefactions (lower pressure) that travel outwards. P-waves (primary seismic waves) are also longitudinal.

Worked example: demonstrating both types with one slinky

Stretch the slinky along the floor with a partner holding the far end.
Transverse: flick your end from side to side, at right angles to the slinky.
Longitudinal: push and pull your end forwards and backwards, along the slinky.

Answer: the direction you move your hand compared with the slinky decides the type of wave

The wave travels, the medium does not

You can show that the medium stays put while the wave moves on:

  • Tie a piece of ribbon to one coil of a slinky. When waves pass, the ribbon moves to and fro (or up and down) but ends up where it started.
  • Drop a pebble into a still pond with a leaf floating on it. Ripples spread out past the leaf, but the leaf only bobs up and down.
  • A loudspeaker playing music does not blow the air in the room away from it – if it did, it would leave a vacuum behind it.

Worked example: exam-style answer (3 marks): explain the motion of a floating ball as waves cross a pond

The ball moves up and down (it oscillates) as each crest and trough passes it. (1)
It does not move along with the waves, because water waves are transverse: the water oscillates at right angles to the direction of energy transfer. (1)
The waves transfer energy across the pond, not water, so the ball stays in the same place on average. (1)

Transverse and longitudinal compared

  • Oscillations compared with energy transfer; Transverse perpendicular (at 90°); Longitudinal parallel (same line)
  • Features of the wave; Transverse peaks (crests) and troughs; Longitudinal compressions and rarefactions
  • Examples; Transverse water ripples, rope waves, EM waves, S-waves; Longitudinal sound, P-waves, push–pull slinky waves
  • Travel through a vacuum?; Transverse only electromagnetic waves; Longitudinal no (sound needs a medium)

Key fact: Electromagnetic waves are transverse. Sound waves are longitudinal. Mechanical waves can be either.

Common mistakes

  • Saying that water waves carry the water along with them. The water (like the floating ball) only moves up and down; the energy moves on.
  • Describing a transverse wave as moving “at 90° to the wave”. Say the oscillations are perpendicular to the direction of energy transfer.
  • Thinking sound is transverse because a sound wave on an oscilloscope screen looks like a wiggly line. The screen shows a graph, not the shape of the wave in the air.
  • Thinking all waves need a medium. Electromagnetic waves travel through a vacuum; only mechanical waves need a medium.

Revision questions

Try these, then check your answers below each one.

Question 1. Which statement describes a transverse wave? (Choose from: The oscillations are perpendicular to the direction of energy transfer., The oscillations are parallel to the direction of energy transfer., The wave carries the medium along with it., The wave can only travel through a vacuum..)

Answer: The oscillations are perpendicular to the direction of energy transfer.

Working: In a transverse wave the oscillations are at right angles (perpendicular) to the direction of energy transfer, e.g. ripples on water.

Question 2. True or false? Sound waves can travel through a vacuum.

Answer: False

Working: False. Sound needs a medium (particles) to pass on its vibrations, so it cannot cross a vacuum such as space.

Question 3. Which of these is a longitudinal wave? (Choose from: P-waves from an earthquake, S-waves from an earthquake, microwaves, ripples on a pond.)

Answer: P-waves from an earthquake

Working: P-waves are longitudinal (push–pull). S-waves, microwaves (electromagnetic) and water ripples are transverse.

Related lessons

Continue in the full course

This lesson is section 12.1 (AGCPH1201) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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