In our article on the properties of waves, we discussed the huge importance of waves for our lives and our technologies.
Our bodies are developed to use sound waves to detect vibration around us and to hear with. Our eyes use light waves to turn a reflection of an object into an image that we understand. Meanwhile, we have developed radio waves – which are a type of light – in order to transmit information remotely, whilst we can now harness the energy from water waves for our society’s use.
Alongside these quite useful waves, there are the seismic waves that are unleashed by earthquakes and volcanoes. The electromagnetic waves that are produced by the interaction of a magnetic field and an electric field. And the waves that are the basic fact of fun things like trampolines and a Mexican wave.
Waves are all around us, in places that we may never have expected. It is therefore so important that we take the time to understand what the hell these things actually are: how they work, their main features and properties, and the main characteristics that their different types exhibit.
The latter is what we are going to be doing here. We’ll focus on the two main types of wave – longitudinal and transverse – and show you the different places in which you can find each. We’ll point you in the direction of surface waves – a combination of the two – whilst we’re here.
We hope you find it interesting!
Firstly, What is a Wave?
Do you remember the definition of a wave? Scientists tend to define it as a disturbance or variation that transmits energy in a regular way. Every part of this definition is important, so let’s take a moment to unpack it.
In mechanical waves – i.e. waves that require a medium through which to pass – the particle matter that facilitates the wave’s energy transfer is disturbed. As the wave’s energy passes through the matter, the particles move and then return to their original position – so that there is a net movement of zero.
It is only energy that is being transmitted in a mechanical wave, then, not mass. But the energy passing through the medium produces oscillations – and these are necessarily regular.
If they were not – if they were just completely random – you would not have a wave. In this case, there would have to be some external source of energy affecting medium that is not the simple travels of energy from point to point.
So, to recap, waves
- are a disturbance in a medium;
- have to be regular in their disturbance; and
- transfer energy from point to point.
A Note on Electromagnetic Waves.
Mechanical waves need a material medium to disturb – otherwise they could not propagate. They transfer energy through material, whether solid, liquid, or gas.
However, there’s another type of wave that we are forgetting – and that’s the electromagnetic wave. These guys – such as visible light, radio waves, ultraviolet, and gamma rays – can happily travel through a space with no material medium. Like a vacuum, or like outer space.
This makes the notion of the disturbance or displacement a bit hard to grasp. Because, how can there be a displacement if there is nothing – literally no material – to displace? Good question – and this has bothered scientists too.
Back in the mid-nineteenth century, James Clerk Maxwell, a scientist famous for what are known as Maxwell’s equations, figured that electromagnetic waves are disturbances that are propagating in the magnetic field. So, they are disturbances just as earthquake waves are, but they happen in a completely different sort of medium to the one that we call ‘matter’.
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Longitudinal Waves and Transverse Waves.
So, now that we have cleared that up, let’s move on to the real content of this article – the nature of and difference between the longitudinal wave and the transverse wave.
Do you have any idea of this difference? If you’ve read our article, What are Waves?, you might well be on the money. Yet, here we’re going to take it in a little more detail.
The difference between longitudinal and transverse waves is all about the wave motion – namely the way in which the waves oscillate. If a wave produces material movement that is perpendicular to the direction of the energy transfer, we call this sort of wave transversal. If the movement is rather parallel to the direction of transferred energy, we call the wave longitudinal.
For clarity, let’s take these one at a time.
What are Transverse Waves?
If there were such a thing as a ‘classic’ wave, it would be the transverse wave. These are the familiar sorts of waves that we study in diagrams. We do this because the transverse waves are the easiest waves to visualise – as they demonstrate a polarization which we can see extending into space.
Think of a skipping rope. As you flick it, a visible wave travels from your hand down the length of the rope. This is a transverse wave.
The scientific definition for a transverse wave is that their displacement of the medium is at right angles to the direction of energy transmission. Thinking of the skipping rope again, that means that the visible wave travels ‘up and down’, the energy travels down the length of the rope.
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Peaks and Troughs.
We call the highest point of a wave – that moment of maximum displacement of the medium – in a transverse wave the ‘peak’ when it is ‘up’ and a ‘trough’ when it is ‘down’.
Consequently, with a transverse wave, we can easily measure the wave’s wavelength as well as its amplitude. ‘Easily’ at least theoretically. Because, again, you can see the distance between the peaks (through which we measure the length of the wave’s oscillation, or its wavelength) as well as the distance between the peaks and the wave’s rest position (the amplitude of the wave).
Both of these – wavelength and amplitude – can tell us the amount of energy being transferred in the wave.
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Examples of Transverse Waves.
We’ve discussed the example of the skipping rope – a type of transversal wave. Yet, there are many more.
A guitar string works in exactly the same way as the skipping rope – except the frequency of the wave is much higher.
Electromagnetic waves such as light and radio waves are also transverse. In their disturbance of the magnetic field, they polarize alternately – meaning when the electric wave is peaking, the magnetic aspect is in a trough.
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What are Longitudinal Waves?
In a transverse wave, the displacement leads to a polarization of the medium. In a longitudinal wave, this disturbance happens in a different way that does not produce these perpendicular peaks and troughs.
In a longitudinal wave, the displacement occurs in the direction of energy’s travel. So, here, rather than a skipping rope, we’re going to have to imagine a slinky – one of those long spirals or springs.
If you were to lay out a slinky flat on a table and shunt one end, what would happen? The parts of its spring would bunch up and extend apart progressively. Rather than the lateral wobble, the energy would transmit within the structure of the slinky itself.
Compressions and Rarefactions.
Scientists call these movements compressions and rarefactions, and they are the longitudinal equivalent to the transverse peaks and troughs.
Compressions are the areas in the medium in which the particles – or the rings of the slinky – are closer together. Here the pressure is very high, which means that the medium can push itself apart again. The rarefaction, meanwhile, are areas of low pressure; they are the areas where the particles of the medium are further apart.
If you were to measure the amplitude or frequency and wavelength of a longitudinal wave, you would take the measurement from the points of highest compression.
Examples of Longitudinal Waves.
So, apart from the slinky (which can, incidentally, also facilitate transverse waves), what other examples of longitudinal waves are there?
You won’t be surprised to hear that there are loads.
Sound waves are perhaps one of the most important – or at least are one of the most convenient examples. They travel through air well, but actually travel faster through liquids and solids – as the medium is denser, meaning that there can be greater compressions of its particles.
What are Surface Waves?
By the way, there is a third type of wave that you might want to know about. These are surface waves – like the famous ocean waves – which play out on the interface between two media.
They are actually the combination of both longitudinal waves and transverse waves. Just so you know!
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