Waves are everywhere. We hear with them, see with them, and they are present in all sorts of different materials and motions.
When you drop a stone into a pool of water, those things that we commonly call ‘ripples’ are better described as waves. When we go to the beach, those big crashing piles of water – waves – are just a larger version of this usually microscopic physical phenomenon. When the wind rushes through trees or over fields, you are seeing waves there too.
Here, we are going to be looking at the nature of waves as defined by physics. We are going to look at some of the features and terms – such as amplitude, wave propagation, frequency and wavelength – to discover what a wave really means to a physicist. And we’re going to look in greater detail at some of the important places in which we find waves in our world – from gravitational and electromagnetic waves to sound waves and water waves.
Because this particular phenomenon is a really crucial part of our world to know. And as soon as you know how to identify a wave – and where you might find them – you’ll see them all over the place.
So, let’s start looking at what a wave actually is. You can find an introduction to waves too.
What Defines a Wave?
You’ve probably seen a diagram of a wave before.
What you see is a fluctuating line that travels over and beneath a central point, in regular intervals. The distance between the highest points on the fluctuating line (or the ‘crests’) are regular, whilst the height and depth of the crests and troughs remain the same too. Without this regularity, you wouldn’t have a wave.
A wave is a disturbance or variation in space-time that is accompanied by a transfer in energy. This is the definition that you will need to remember. If you imagine that, in an ideal world, a normal wave line would be completely flat – with no wave at all – what you see on the diagram is a transfer of energy that is the disturbance.
Of course, this isn’t a reality that we ever witness. Energy is always being transferred – and, as such, there are always waves present, in much more complex variations and interferences than any diagram could possibly show.
Mechanical Waves and Electromagnetic Waves.
Before we go on to tackle the shapes and movements of different waves, it is important to recognise two prior types of waves. Because maybe you have heard of electromagnetic waves – which are light waves by the way. These function in a slightly different way to the waves in our diagram above.
Mechanical waves are the waves that we recognise all around us. These are the ocean waves and the ripples of water, the sound waves with which we hear, and the seismic waves that destroy buildings and rupture the earth in an earthquake.
These mechanical waves need a medium through which to travel – as in, some sort of matter. In these guys, energy is moved across particles.
Imagine an earthquake. Here, an initial force causes disturbances across the matter of the earth. This force can travel so far – as in, damage can be done hundreds of miles from the epicentre – because the waves can travel through all the matter of the earth, transporting kinetic energy for huge distances.
Without the earth’s substance – or if, say, for some reason, the seismic waves encountered a vacuum – the earthquake could not possibly pass through.
Whilst mechanical waves need a medium to disturb in order to transfer energy, electromagnetic waves do not. These, such as light waves, can propagate even across a vacuum. And this fact is a little bit problematic for our understanding of waves per se.
You’ve probably heard of the famous scientific problem that states that light is both a wave and a particle (you may well have heard of Schrodinger’s Cat) – that, depending on how you try to observe it, light behaves both like a particle and like a wave. This is one of the complex things about it.
Electromagnetic waves like light are produced by the interaction of a magnetic field and an electric field. Changes to one of these produces changes in the other – and at the same time produces electromagnetic waves.
Alongside light, radio waves are another type of electromagnetic wave.
Waves and Energy Transfer.
Having clarified this, let’s pin down the most important – and useful – part of the nature and structure of waves. This is the fact that they transfer energy.
In waves, it is energy that is transferred. Whilst the particles or matter through which the waves pass briefly moves, the net movement of material is zero: the particles return to their original position after the wave has passed on.
Remember: energy, not matter, is transferred by waves.
Longitudinal, Transverse, and Surface: What are the Different Types of Waves?
Alongside these two fundamental types of waves, there are different categorisations that describe the type of movements that the waves make. These categories describe the ways in which particles are displaced by the wave – and the different materials through which the wave propagates.
We describe waves as longitudinal if they have particles that move parallel to the movement of energy in the wave. Rather than the crests and troughs that we see in the classic wave diagram, longitudinal waves do not have this up-down motion.
This, in scientific terms, is expressed differently. They do not demonstrate polarization – i.e. they don’t have the peaks and troughs – but rather their oscillation is in the direction of the energy’s travel.
Sound waves are an example of this sort of wave – and they can move through solids, liquids, and gases.
A transverse wave is the type of wave that we see in our familiar wave diagram. Here, the movement of particles is at right angles – it is perpendicular – to the movement of energy.
A transverse wave demonstrates the wave polarization that a longitudinal wave would lack: they have that clear movement between peak and trough. This polarisation, by the way, is measured in amplitude, which describes the distance between the peaks and the centre of the wave.
These are the easiest waves to study because you can easily see the polarisation and the wavelength – or the distance of an oscillation (the time it takes for a wave to repeat itself).
Imagine a rope or a slinky that you raise and lower rapidly. Across the length of the rope you will see a peak travelling along the length of the rope. This is a transverse wave.
In surface waves, particles travel in a circular motion spreading out from the originary disturbance. These are actually a combination of transverse and longitudinal waves that play along the interface between different media.
You’ll know these types of waves: drop a stone in a pool of water and every ripple that you will see is a surface wave. Their circular motion comes from this combination of transverse and longitudinal.
Find out more about transverse and longitudinal waves!
What are the Different Parts of a Wave?
Let’s do a little recap of the most important parts of the wave that we have so far discussed. If you are reading this article to help with school work, it is important that you know all of these terms very well.
So, here are the most important parts of a wave.
- Rest position – This is the position of particles when there is no energy or wave passing through them; these particles are undisturbed. On a wave diagram, this is the line that is drawn through the centre of the wave.
- Displacement – This is the distance that a particle has moved from its rest position due to its disturbance.
- Amplitude – The measurement of displacement, we call the amplitude the maximum disturbance of a particular point in the medium. This is the distance between the rest position and the highest peak or trough.
- Peaks and troughs – The points of greatest disturbance or maximum displacement, above and below the rest position.
- Oscillation – The repetition of a wave – i.e. the space from peak to peak.
- Wavelength – The actual distance covered by one oscillation – usually measured from peak to peak.
- Frequency or wave speed – The number of times a wave oscillates in a second.