Transverse & Longitudinal Waves (AQA GCSE Physics: Combined Science)

• Waves are repeated vibrations that transfer energy
• Energy is transferred by parts of the wave knocking nearby parts
  • This is similar to the effect of people knocking into one another in a crowd, or a "Mexican Wave" at football matches

• Waves can exist as one of two types:
  • Transverse
  • Longitudinal

Transverse Waves

• Transverse waves are defined as:

Waves where the points along its length vibrate at 90 degrees to the direction of energy transfer

• For a transverse wave:
  • The energy transfer is perpendicular to wave motion
  • They transfer energy, but not the particles of the medium
  • They can move in solids and on the surfaces of liquids but not inside liquids or gases
  • Some transverse waves (electromagnetic waves) can move in solids, liquids and gases and in a vacuum
    

• The point on the wave that is:
  • The highest above the rest position is called the peak, or crest
  • The lowest below the rest position is called the trough

Transverse waves can be seen in a rope when it is moved quickly up and down

• Examples of transverse waves are:
  • Ripples on the surface of water
  • Vibrations in a guitar string
  • S-waves (a type of seismic wave)
  • Electromagnetic waves (such as radio, light, X-rays etc)

Representing Transverse Waves

• Transverse waves are drawn as a single continuous line, usually with a central line showing the undisturbed position
• The curves are drawn so that they are perpendicular to the direction of energy transfer
  • These represent the peaks and troughs

Transverse waves are represented as a continuous solid line

• Longitudinal waves are defined as:

Waves where the points along its length vibrate parallel to the direction of energy transfer

• For a longitudinal wave:
  • The energy transfer is in the same direction as the wave motion
  • They transfer energy, but not the particles of the medium
  • They can move in solids, liquids and gases
  • They can not move in a vacuum (since there are no particles)

• The key features of a longitudinal wave are where the points are:
  • Close together, called compressions
  • Spaced apart, called rarefactions

Longitudinal waves can be seen in a slinky spring when it is moved quickly backwards and forwards

• Examples of longitudinal waves are:
  • Sound waves
  • P-waves (a type of seismic wave)
  • Pressure waves caused by repeated movements in a liquid or gas

Representing Longitudinal Waves

• Longitudinal waves are usually drawn as several lines to show that the wave is moving parallel to the direction of energy transfer
  • Drawing the lines closer together represents the compressions
  • Drawing the lines further apart represents the rarefactions

Longitudinal waves are represented as sets of lines with rarefactions and compressions

• Their different motions can be shown on ropes (transverse) and springs (longitudinal)

Waves can be shown through vibrations in ropes or springs

• The different properties of transverse and longitudinal waves are shown in the table:

Transverse Waves v Longitudinal Waves Table

Wavefronts

• Both transverse and longitudinal waves can be represented as wavefronts
  • This is where the waves are viewed from above

• For a transverse wave:
  • One line represents either a peak or a trough

• For a longitudinal wave:
  • One line represents either a compression or a rarefaction

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

Step 1: Identify the type of wave

• • The type of wave on the surface of a body of water is a transverse wave
  • This is because the duck is moving perpendicular to the direction of the wave

Step 2: Describe the motion of the toy duck

• • 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

• • Both transverse and longitudinal waves transfer energy, but not the particles of the medium
  • 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
  • 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