A slinky spring can be used to demonstrate both transverse and longitudinal waves. Fig. 16.5 shows how each can be created.
• In Fig. 16.5a, the slinky is vibrated up–down to create a transverse wave. The wave travels from side to side but the vibration is up– down.
• In Fig. 16.5b, the slinky is compressed and extended forward and back and this causes a pulse to move from side to side, as well as the vibration pulsing from side to side parallel to wave motion. This creates a longitudinal wave.
Analysing waves b compression a wave direction spring moves up and down
wave direction
spring moves back and forth
rarefaction
FIG. 16.5 Demonstrating transverse and longitudinal waves
The terms used to describe waves differ slightly depending on whether you are talking about longitudinal or transverse waves.
Fig. 16.6 shows each type of wave and their respective terms. For ease of comparison, it is usually taken that the crest of a transverse wave is equivalent to the compression of a longitudinal wave.
• Wavelength: This is the length of a complete cycle. It can be taken at any point on a wave to its next corresponding point. In the diagrams, it is from crest to crest in the transverse wave (Fig. 16.6a) and from compression to compression in the longitudinal wave (Fig. 16.6b). Its symbol is λ (lambda) and it is measured in metres. a
• Amplitude: This is the maximum displacement from the mean position. It is taken as the distance between the base line and the crest or trough in the case of a transverse wave. It is measured in metres.
• Frequency: This is the number of vibrations that occur per second. Its symbol is f and it is measured in hertz (Hz).
• Speed: This is the speed at which the wave propagates through space or a medium. Its symbol can be c. It is measured in metres per second (m s–1).
• Period: This is the time taken for one complete cycle to occur. It is measured in seconds. Its symbol is T. It is also the
reciprocal of frequency
( T = 1 __
f ) . b wavelength wavelength crest amplitude
trough wave direction
compression FIG. 16.6 Illustrating the terms used to describe waves
