Features of Waves (Cambridge International IGCSE Physics)

Waves & energy transfer

Spec point — 3.1.1 · Know that waves transfer energy without transferring matter

• Waves transfer energy without transferring matter

  • For sound waves, this means it is the wave and not the air molecules (the matter) itself that travels

• Objects floating on water provide evidence that waves only transfer energy and not matter

  • It is possible to see objects on the surface of the water bob up and down but not change their position
  • This is because it is the wave, and not the water (the matter), that travels

• Waves are described as oscillations or vibrations about a fixed point

  • For example, ripples cause particles of water to oscillate up and down
  • Sound waves cause particles of air to vibrate back and forth

Worked Example The diagram below shows a toy duck bobbing up and down on top of the surface of some water, as waves pass it underneath.

Use this image

Explain how the toy duck demonstrates that waves do not transfer matter.

[2]

**Answer:

• The plastic duck moves up and down but does not travel with the wave along the surface of the water
• The water waves transfer energy, but the water particles only oscillate up and down about a fixed position — they do not travel along with the wave [1 mark]
• This means when a wave travels between two points, no matter travels with it and the points on the wave vibrate up and down about fixed positions** [1 mark]

Exam Hint There is a key distinction between the particles (or oscillations) of a wave and the wave itself.

The motion of the wave causes the particles to move. The particles themselves are not the wave.

Wave motion

Spec point — 3.1.1 · Describe what is meant by wave motion as illustrated by vibrations in ropes and springs, and by experiments using water waves

• Wave motion can be illustrated by:

  • vibrations in ropes and springs
  • experiments using water waves

Waves can be shown through vibrations in ropes or springs

Use this image

• Properties of waves can be observed using water waves in a ripple tank

  • These properties include frequency, wavelength, amplitude and wave speed

Wave motion of water waves may be demonstrated using a ripple tank

Use this image

Features of a wave

Spec point — 3.1.1 · Describe the features of a wave in terms of wavefront, wavelength, frequency, crest (peak), trough, amplitude and wave speed

• When describing wave motion, there are several terms which are important to know, including:

  • wavefront
  • amplitude
  • wavelength
  • frequency
  • crest (peak)
  • trough
  • wave speed

Wavefront

• Wavefronts are a useful way of picturing waves from above

• Each wavefront, drawn as a single line, joins points on the wave that are in step with each other, which in practice are usually the crests

• The image below illustrates how wavefronts are visualised:

  • The arrow shows the direction the wave is moving and is sometimes called a ray
  • The space between each wavefront represents the wavelength
  • When the wavefronts are close together, this represents a wave with a short wavelength
  • When the wavefronts are far apart, this represents a wave with a long wavelength

Wavefronts as viewed from above

Diagram showing a wave moving to the right, drawn as a series of wavefronts

Use this image

Amplitude

• Amplitude is defined as:

The maximum displacement of a wave from its undisturbed position

• It is given the symbol $A$ and is measured in metres (m)
• On a graph where the vertical axis is displacement, amplitude is measured from the undisturbed position to either the highest point of the wave (peak) or the lowest point (trough)

Wavelength

• Wavelength is defined as:

The distance from one point on the wave to the same point on the next wave

• In a transverse wave:

  • The wavelength can be measured from one peak to the next peak

• In a longitudinal wave:

  • The wavelength can be measured from the centre of one compression to the centre of the next

• Wavelength is given the symbol $λ$ (lambda) and is measured in metres (m)

• On a graph where the horizontal axis is distance, the wavelength can be determined by measuring the distance from one point on the wave to the same point on the next wave

Using a graph to determine wavelength and amplitude

Diagram showing the amplitude and wavelength of a wave

Use this image

Frequency

• Frequency is defined as:

The number of waves passing a point in a second

• Frequency is given the symbol $f$ and is measured in hertz (Hz)

Crests and troughs

• A crest, or a peak, is defined as:

The highest point on a wave above its undisturbed position

• A trough is defined as:

The lowest point on a wave below its undisturbed position

Wave crests and troughs

Diagram showing a crest and a trough on a transverse wave

Use this image

Wave speed

• Wave speed is the speed at which energy is transferred through a medium
• Wave speed is defined as:

The distance travelled by a wave each second

• The equation used to calculate wave speed is explained in The wave equation

Worked Example Small water waves are created in a ripple tank by a wooden bar. The wooden bar vibrates up and down hitting the surface of the water. The diagram below shows a cross-section of the ripple tank and water.

Use this image

Identify the letter which shows:

a) the amplitude of a water wave. [1]

b) the wavelength of the water wave. [1]

Answer:

Part (a)

Step 1: Recall the definition of amplitude

• Amplitude = The distance from the undisturbed position to the peak or trough of a wave

Step 2: Mark the undisturbed position on the wave

• This is the centre of the wave

Use this image

Step 3: Identify the arrow between the undisturbed position and a peak

• The amplitude is shown by arrow **D [1 mark]

Part (b)

Step 1: Recall the definition of wavelength

• Wavelength = The distance from one point on the wave to the same point on the next wave

Step 2: Draw lines on each horizontal arrow

• This helps to identify the points on the wave the arrows are referring to

Use this image

Step 3: Identify the arrow between two of the same points on the wave

• The wavelength is shown by arrow** C [1 mark]

Exam Hint Make sure you understand how to identify the wavelength and amplitude of a wave from a diagram. On a displacement-distance graph, the wavelength is the distance from one point on the wave to the same point on the next wave (e.g. crest to crest), while amplitude is the distance from its undisturbed position to either a peak or trough (not the crest to trough distance).