3. WAVES
3.1 General Properties of Waves
3.1.1 Features of Waves
Waves - Basic
Waves transfer energy and information
Waves are described as oscillations or vibrations about a fixed point
o For example, ripples cause particles of water to oscillate up and down
o Sound waves cause particles of air to vibrate back and forth
In all cases, waves transfer energy without transferring matter
o For water waves, this means it is the wave and not the water (the matter) itself
that travels
o 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
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.
Explain how the toy duck demonstrates that waves do not transfer matter.
Step 1: Identify the type of wave
o The type of wave on the surface of a body of water is a transverse wave
o This is because the duck is moving perpendicular to the direction of the wave
Step 2: Describe the motion of the toy duck
o The plastic duck moves up and down but does not travel with the wave
Step 3: Explain how this motion demonstrates that waves do not transfer matter
o Both transverse and longitudinal waves transfer energy, but not the particles of the
medium
o This means when a wave travels between two points, no matter actually travels
with it, the points on the wave just vibrate back and forth about fixed positions
o Objects floating on the water simply bob up and down when waves pass under
them, demonstrating that there is no movement of matter in the direction of the
wave, only energy
Exam Tip
There is a key distinction between the wave itself and the particles in the wave. The wave causes
the particles to move, but the particles themselves are not the wave, since they are not transferring
energy, which a wave does.
Wave Motion
Wave vibrations can be shown on ropes (transverse) and springs (longitudinal)
, Waves can be shown through vibrations in ropes or springs
Demonstrating Wave Motion
Properties of waves, such as frequency, wavelength and wave speed, can be observed
using water waves in a ripple tank
Wave motion of water waves may be demonstrated using a ripple tank
The wavelength of the waves can be determined by:
o Using a ruler to measure the length of the screen
o Dividing this distance by the number of wavefronts
The frequency can be determined by:
o Timing how long it takes for a given number of waves to pass a particular point
o Dividing the number of wavefronts by the time taken
, The wave speed can then be determined by:
o Using the equation wave speed = frequency × wavelength
Features of a Wave
When describing wave motion, there are several terms which are important to know,
including:
o Crest (Peak)
o Trough
o Amplitude
o Wavelength
o Frequency
o Wave speed
o Wavefront
Crests & Troughs
A crest, or a peak, is defined as:
The highest point on a wave above the equilibrium, or rest, position
A trough is defined as
The lowest point on a wave below the equilibrium, or rest, position
Diagram showing a crest and a trough on a transverse wave
Amplitude
Amplitude is defined as:
The distance from the undisturbed position to the peak or trough of a wave
It is given the symbol A and is measured in metres (m)
Amplitude is the maximum or minimum displacement from the undisturbed position
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:
o The wavelength can be measured from one peak to the next peak
In a longitudinal wave
o The wavelength can be measured from the centre of one compression to the centre
of the next
The wavelength is given the symbol λ (lambda) and is measured in metres (m)
The distance along a wave is typically put on the x-axis of a wave diagram
, Diagram showing the amplitude and wavelength of a wave
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)
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
Wave speed is given the symbol, ν, and is measured in metres per second (m/s), it can be
calculated using:
wave speed = frequency × wavelength
Wavefront
Wavefronts are a useful way of picturing waves from above: each wavefront is used to
represent a single wave
The image below illustrates how wavefronts are visualised:
o The arrow shows the direction the wave is moving and is sometimes called a ray
o The space between each wavefront represents the wavelength
o When the wavefronts are close together, this represents a wave with a short
wavelength
o When the wavefronts are far apart, this represents a wave with a long wavelength
Diagram showing a wave moving to the right, drawn as a series of wavefronts
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.
3.1 General Properties of Waves
3.1.1 Features of Waves
Waves - Basic
Waves transfer energy and information
Waves are described as oscillations or vibrations about a fixed point
o For example, ripples cause particles of water to oscillate up and down
o Sound waves cause particles of air to vibrate back and forth
In all cases, waves transfer energy without transferring matter
o For water waves, this means it is the wave and not the water (the matter) itself
that travels
o 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
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.
Explain how the toy duck demonstrates that waves do not transfer matter.
Step 1: Identify the type of wave
o The type of wave on the surface of a body of water is a transverse wave
o This is because the duck is moving perpendicular to the direction of the wave
Step 2: Describe the motion of the toy duck
o The plastic duck moves up and down but does not travel with the wave
Step 3: Explain how this motion demonstrates that waves do not transfer matter
o Both transverse and longitudinal waves transfer energy, but not the particles of the
medium
o This means when a wave travels between two points, no matter actually travels
with it, the points on the wave just vibrate back and forth about fixed positions
o Objects floating on the water simply bob up and down when waves pass under
them, demonstrating that there is no movement of matter in the direction of the
wave, only energy
Exam Tip
There is a key distinction between the wave itself and the particles in the wave. The wave causes
the particles to move, but the particles themselves are not the wave, since they are not transferring
energy, which a wave does.
Wave Motion
Wave vibrations can be shown on ropes (transverse) and springs (longitudinal)
, Waves can be shown through vibrations in ropes or springs
Demonstrating Wave Motion
Properties of waves, such as frequency, wavelength and wave speed, can be observed
using water waves in a ripple tank
Wave motion of water waves may be demonstrated using a ripple tank
The wavelength of the waves can be determined by:
o Using a ruler to measure the length of the screen
o Dividing this distance by the number of wavefronts
The frequency can be determined by:
o Timing how long it takes for a given number of waves to pass a particular point
o Dividing the number of wavefronts by the time taken
, The wave speed can then be determined by:
o Using the equation wave speed = frequency × wavelength
Features of a Wave
When describing wave motion, there are several terms which are important to know,
including:
o Crest (Peak)
o Trough
o Amplitude
o Wavelength
o Frequency
o Wave speed
o Wavefront
Crests & Troughs
A crest, or a peak, is defined as:
The highest point on a wave above the equilibrium, or rest, position
A trough is defined as
The lowest point on a wave below the equilibrium, or rest, position
Diagram showing a crest and a trough on a transverse wave
Amplitude
Amplitude is defined as:
The distance from the undisturbed position to the peak or trough of a wave
It is given the symbol A and is measured in metres (m)
Amplitude is the maximum or minimum displacement from the undisturbed position
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:
o The wavelength can be measured from one peak to the next peak
In a longitudinal wave
o The wavelength can be measured from the centre of one compression to the centre
of the next
The wavelength is given the symbol λ (lambda) and is measured in metres (m)
The distance along a wave is typically put on the x-axis of a wave diagram
, Diagram showing the amplitude and wavelength of a wave
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)
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
Wave speed is given the symbol, ν, and is measured in metres per second (m/s), it can be
calculated using:
wave speed = frequency × wavelength
Wavefront
Wavefronts are a useful way of picturing waves from above: each wavefront is used to
represent a single wave
The image below illustrates how wavefronts are visualised:
o The arrow shows the direction the wave is moving and is sometimes called a ray
o The space between each wavefront represents the wavelength
o When the wavefronts are close together, this represents a wave with a short
wavelength
o When the wavefronts are far apart, this represents a wave with a long wavelength
Diagram showing a wave moving to the right, drawn as a series of wavefronts
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.
