Waves and energy – wave basics
What is a wave? Most of us think of the waves we see at the beach. The waves come in from out in the ocean and crash against the shore. If we consider how the water is actually moving in a wave, the situation becomes quite interesting. Although the water is in motion, the wave is travelling across the water, and if we could examine a small drop of water, we would find the water is actually moving mostly up and down.
Water waves are quite complicated, so let’s begin by examining waves in a simpler system – a slinky.
Slinky
A slinky provides a simple system for the modelling of waves.
Transverse and longitudinal waves
There are two ways to make waves on a slinky. The first is called a transverse wave. In a transverse wave, the material of the slinky moves in one direction (up and down) while the wave travels along the slinky in another direction. The second type of wave is called a longitudinal wave. In a longitudinal wave, the slinky moves back and forth, and the wave moves in the same direction as the slinky is moving.
This video shows examples of both waves. Watch a particular point on the slinky as each type of wave passes.
Examples of transverse and longitudinal waves
This video shows one wave moving from left to right. Watch the left-hand side of the video as the wave enters the screen. The transverse wave begins by going up, then going down and finally comes back to the starting position. The longitudinal wave begins by pulling back and then pushing forward.
Note: There is no sound on this video.
Acknowledgements
Dr Michael R Gallis, Animations for Physics and Astronomy, Penn State Schuylkill, CC BY-NC 3.0
This video shows a series of waves moving along the slinky.
A series of transverse and longitudinal waves
The video shows a series of transverse and longitudinal waves. Watch a particular point on the slinky as each type of wave passes.
Note: There is no sound on this video
Acknowledgements
Dr Michael R Gallis, Animations for Physics and Astronomy, Penn State Schuylkill, CC BY-NC 3.0
Real-world examples of transverse waves are the strings of musical instruments, light waves and water waves. Examples of longitudinal waves are sound waves and the P-waves in earthquakes.
Illustrating transverse and longitudinal waves
Most illustrations of waves show transverse waves, primarily because they are easier to visualise.
Longitudinal air particle wave
An actual sound wave in air may look something like this diagram. The dots represent air particles, showing areas of dense air and areas of less dense air.
Since longitudinal waves are difficult to draw, most waves are drawn as transverse waves. The high points of the waves are the peaks and represent more dense areas, and the low points are troughs, representing less dense areas.
Transverse longitudinal wave
Most waves are drawn as transverse waves. The high points of the transverse waves (peaks) represent more dense areas of the longitudinal waves, and the low points (troughs) represent less dense areas.
Wavelength, frequency and amplitude
The three terms used when describing a wave are wavelength, amplitude and frequency.
Wavelength is the length of one wave. Waves come in all different lengths. Sound waves vary in length from 2 centimetres (a very high note) to nearly 20 metres (a very low note). Light waves vary in length from 0.0004 millimetres (violet) to 0.0007 millimetres (red).
Describing waves
The three terms used when describing a wave are: wavelength (the length of one wave), amplitude (the height of a wave from equilibrium position to peak) and frequency, (the number of waves that pass a point in one second).
Amplitude describes the height of waves from the starting position to the top of the wave. In the case of a transverse wave, it describes how much the material of the wave is compressed or expanded. The amplitude of a sound wave is its volume or loudness. The amplitude of a light wave is its brightness.
Frequency is how many waves pass a certain point in one second, measured in hertz (Hz). In music, middle C is about 262 Hz. This means that, if we could see the waves passing when someone was playing middle C, we would see about 262 waves pass our eyes in each second. The wavelength of middle C is about 132 centimetres. Human hearing ranges from approximately 20 Hz to 20,000 Hz, depending on age.
The frequency of a sound is commonly referred to as its pitch. High-frequency notes are high notes, and low frequency notes are low notes. Frequency in light describes the colour of the light. Red light has a relatively low frequency, and violet light has a high frequency.
The article Waves and energy – energy transfer discusses how waves transmit energy.
Nature of science
There are many words – like wave – that have an everyday meaning and a scientific meaning. Discussion about the different meanings helps to avoid confusion and alternative conceptions. Developing the knowledge of scientific vocabulary is part of the ‘Communicating in science’ achievement aim.
Activity ideas
In the activity Modelling waves with slinkies, students model how sound travels by sending waves along two stretched plastic slinkies tied together.
In the activity Sound on an oscilloscope, students play different kinds of sounds near a computer microphone and watch the resulting visual display created by oscilloscope software.
Related collection
The Science Learning Hub team has curated an introductory collection of resources to help with teaching waves. Login to make this collection part of your private collection, just click on the copy icon . You can then add additional content, notes and make other changes. Registering an account for the Science Learning Hubs is easy and free – sign up with your email address or Google account. Look for the Sign up button at the top of each page.
Useful link
Use this University of Colorado simulation to experiment with different types waves on a string.
