Graphical Representations of Transverse & Longitudinal Waves (OCR A Level Physics)
Revision Note
Graphical Representations of Transverse & Longitudinal Waves
Transverse and longitudinal waves can be represented graphically
Transverse Waves
Transverse waves show areas of crests (peaks) and troughs
Diagram of a transverse wave
The peaks are the maximum positive displacements
The troughs are the maximum negative displacements
The direction of the energy transfer is perpendicular to the direction of vibration of the particles in the wave
Longitudinal Waves
Longitudinal waves show areas of compressions and rarefactions
Diagram of a longitudinal wave
The compressions are areas of high pressure due to particles being close together.
The rarefactions are areas of low pressure due to the particles spread further apart
The direction of energy transfer is parallel to the direction of vibration of the particles in the wave
Worked Example
The graph shows how the displacement of a particle in a wave varies with time.
Which statement is correct?
A. The wave has an amplitude of 2 cm and could be either transverse or longitudinal.
B. The wave has an amplitude of 2 cm and has a time period of 6 s.
C. The wave has an amplitude of 4 cm and has a time period of 4 s.
D. The wave has an amplitude of 4 cm and must be transverse.
Answer: A
Examiner Tips and Tricks
Both transverse and longitudinal waves can look like transverse waves when plotted on a graph - make sure you read the question and look for whether the wave travels parallel (longitudinal) or perpendicular (transverse) to the direction of travel to confirm which type of wave it is.
Demonstrating Waves Using a Ripple Tank
Waves can also be demonstrated by ripple tanks
These produce a combination of transverse and longitudinal waves
Wave effects can be demonstrated using a ripple tank
In a ripple tank, a motorised wooden straight-edged bar produces plane (straight) waves while a small dipper produces circular waves
When a light is shone from above, the bright bands seen on the screen below the tank show the wave crests (wavefronts)
This makes it possible to measure the wavelength of the water waves and investigate the angles of reflection and refraction
Reflection can be investigated using plane and curved surfaces, and the angles of incidence and reflection measured with respect to the normal
Refraction can be investigated using a glass sheet to decrease the water depth and produce a region with a different wave speed
If the separation of the wavefronts decreases, this shows they are travelling more slowly and vice versa
Changing the angle of the wooden bar causes the wavefronts to go in a different direction
The ripple tank, therefore, can also be used to study interference and diffraction
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