Without waves there would be nothing. No heat, no light, no sound. Not even any movement. Not a single thing.
Because waves aren’t just some abstract scientific concept – or something that you only see on the surface of the ocean. They are literally everywhere at all times.
Waves, quite simply, are disturbances or variations in a medium that allow the transfer of energy. Without waves, energy cannot do anything. And without energy, waves wouldn’t be able to create a disturbance or displacement – and so wouldn’t be at all.
These things that we call waves, therefore, are a defining feature of our universe. And they help to explain a lot of the phenomena of physics – from light, which is a type of electromagnetic wave, to sound, which is one of many mechanical waves. But radio waves, x-rays, heat – not to mention ocean waves, the movement of a rope, and the vibration of a guitar string – are all the results of the same.
So, don’t be one of those people that thinks waves are irrelevant to your life – or that science is ‘boring’. Because, without these things, we’d be nothing.
Here, instead, is a guide to the most important aspects of waves and their behaviour – from their physical properties to some of the technologies in which they are used.
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What are the Properties of Waves?
Again, waves are disturbances in a medium that come with a transfer of energy. This is an important point to remember: waves transfer energy, not mass.
Imagine a train track. If you were to lay your ear to a train track, you would be able to hear someone knocking on the train from a great distance. However, in this case, it is only the sound energy that is being transferred: every time the other person knocks, you are not being hit by the mass of the track, just the sound energy.
The same happens with all different types of wave: it is not mass that is transferred, it is the energy.
However, waves displace mass as they propagate. And the way in which they displace this mass determines whether they will be categorised as a transverse wave or a longitudinal wave. Where, in the former, mass is displaced at a perpendicular angle to the direction of the energy’s movement, in the latter, displacement is parallel to that movement.
Amplitude and Wavelength.
Regardless of the movement of the medium’s displacement, both types of waves come in different amplitudes and with different wavelengths. These are the two central ways in which we measure waves in general.
Amplitude refers to the magnitude of displacement. This would be seen, on a diagram of a transverse wave, by the polarization, or the distance between the peak or trough of a wave and the rest position. The amplitude of a wave would determine the loudness of a sound wave or the strength of a seismic wave.
Frequency, on the other hand, is the measurement of the speed of the wave. This is measured by reference to the number of oscillations – the repetition of a wave – per second and is measured in Hertz.
A sound wave with a greater frequency would be of a higher pitch, for example.
Whilst it is common to say that waves need a material medium through which to travel, this is not strictly true. Only mechanical waves need a medium of molecules and atoms; only these cannot travel in a vacuum.
Electromagnetic waves are, on the other hand, self-propagating. These are able to travel through a vacuum because the medium that they are disturbing or displacing is not strictly matter. Their disturbances are actually to the electromagnetic field which they themselves create.
For more, check out our article on the properties of waves.
Transverse Waves and Longitudinal Waves.
We mentioned above the difference between transverse waves and longitudinal waves. Whilst the latter displaces its medium parallel to the direction of energy’s travel, in the former the displacement is perpendicular to this movement.
This makes these things work in slightly different ways – and makes their terminology slightly different too.
In transverse waves, we talk about peaks and troughs on a wave diagram – these being the moments of greatest displacement in the wave or furthest parts of the wave from its rest position. Through variations in the pressure between particles as the energy moves through them, they move outwards from the rest position and back in again.
However, in longitudinal waves, all of the movement is parallel and the variations in pressure occur in the direction of the energy’s travel. Rather than peaks and troughs, then, we have compressions (areas of high pressure) and rarefactions (areas of low pressure).
Examples of Transverse and Longitudinal Waves.
It’s worth remembering some of the more ‘famous’ examples of transverse and longitudinal waves – as these help you to remember the differences between them.
So, what are examples of transverse waves? The vibrations on a guitar string are transverse waves, as are those that you might make as you wobble one end of a skipping rope.
Light waves are also transverse – along with their associated waves like radio waves and all the waves of electromagnetism.
Longitudinal waves are those you might see if you were to stretch a slinky across a table and shunt one end of it. You’d be able to see the compressions and rarefactions in the coils of the slinky.
Sound waves too are longitudinal, by the way – and they can travel through liquids, gases, and solids.
Find out more in our article on transverse and longitudinal waves.
The Science of Reflection and Refraction.
We’ve discussed the nature and types of wave. However, let’s take a closer look at their behaviour.
One of the most interesting aspects of waves is what happens when they encounter different media on their travels. What happens to an airborne wave when it hits a liquid? Or what happens when it hits a solid? Or even, what happens if a wave in a solid hits a different solid of a different density?
There are actually plenty of options for waves in this situation. And the actual answer brings in lots and lots of different variables – from the wave’s wavelength and amplitude to the nature of the interface between the two media, from the wave’s angle of incidence to the chemical makeup of the different media.
What Happens when a Wave Meets an Interface?
An interface is one of the most important moments in a wave’s busy little life. Because here it has a number of options - of which we'll discuss here only one.
It can be reflected. In reflection, the wave bounces off the interface and returns into the medium from which it came. This is due to the wave having a different frequency to the vibration of the electrons on the surface of the new medium.
However, this can either result in specular reflection or diffuse reflection. In the former, you would get a mirror-like effect, as the waves all reflect in the same direction. If the reflection is diffuse, however, you rather get a situation like looking at a wall. You don’t see a conventional reflection of an image, yet the light has nonetheless bounced off it.
Find out more about reflection in our article on reflection and refraction.
What are Sound and Ultrasound?
We hear sounds all the time all around us. Pour a glass of water and there is sound or take a step and there is sound again.
Sound is also something that is produced by waves. Or, better, it is a type of wave that we recognise to be sound.
What we call sound is a whole series of vibrations that are the result of energy that is propagating through material. Sound waves are longitudinal waves that propagate from an original disturbance – and they come in all different frequencies, wavelengths, and amplitudes that are responsible for their pitch, volume, and tone.
The greater the amplitude the louder the sound – whilst the higher the frequency the higher the pitch.
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What is the Difference between Sound Waves and Ultrasound?
Yet, there are frequencies of sound that we cannot hear at all. Those types of waves we call ultrasound – which is actually the vast majority of sound waves.
Dogs can hear some of the frequencies we call ultrasound – but this only points to the fact that there is no real difference between the two.
Find out more about ultrasound and sound in our article.