If you have been tracking our articles on the science of waves and their properties, you will know that they play a significant role in just about every area of life.
Most remarkably human bodies are designed to use sound waves for detecting vibrations and hearing, while our eyes rely on light waves to transform reflections into understandable images. In addition, we've engineered radio waves, a form of light, for remote information transmission, and even harnessed the energy from water waves for all kinds of benefits for society.
From small to seismic, waves can also be generated by earthquakes and volcanoes, electromagnetic waves and even playful waves like the Mexican wave.
Waves permeate unexpected aspects of our surroundings and emphasise the importance of comprehending their nature—how they function, their key attributes, and the distinct characteristics of various types. In this article, our focus shifts to how to explain the fundamental aspects of waves, particularly transverse and longitudinal waves. We'll also explore where each type can be encountered and introduce you to surface waves, a combination of the two.
Firstly, What is a Wave?
Do you recall the definition of a wave?
Scientists typically describe it as a disturbance or variation that systematically transmits energy.
Each element of this definition holds significance, so let's take a moment to unpack it.
In the realm of mechanical waves—those requiring a medium to propagate—the particles in the medium facilitating the transfer of the wave's energy undergo disturbance. As the wave's energy traverses through the medium, the particles move and subsequently return to their original positions, resulting in a net movement of zero.
In a mechanical wave, it's important to note that only energy is being transmitted, not mass.
However, the energy coursing through the medium gives rise to oscillations, and these oscillations are inherently regular. If they were not regular, if they were entirely random, a wave wouldn't be formed. In such a scenario, there would need to be an external source of energy that affects the medium. This is distinct from the simple transmission of energy from one point to another.
In summary, waves:
- Are a disturbance in a medium;
- Must exhibit regularity in their disturbance; and
- Transfer energy from point to point.
A Note on Electromagnetic Waves
Mechanical waves necessitate a material medium for propagation; without it, they cannot advance. These waves transfer energy through materials, be they solid, liquid, or gas.
Yet, there's another category of waves we shouldn't overlook—electromagnetic waves.
Examples include visible light, radio waves, ultraviolet, and gamma rays, all of which can traverse space that is devoid of a material medium, much like the vacuum of outer space.
Understanding the concept of disturbance or displacement becomes challenging in this context.
How can there be displacement when there is literally no material to displace? If you can’t answer that, don’t worry, it is a good question that has puzzled scientists forever.
In the mid-nineteenth century, James Clerk Maxwell, renowned for Maxwell's equations, proposed that electromagnetic waves are disturbances that propagate in the magnetic field. These disturbances, much like earthquake waves, occur in a medium that is entirely distinct from what we conventionally define as 'matter.'
Transverse Waves and Longitudinal Waves
Now that we've clarified that point, let's delve into the focus of this article—the nature of, and the distinctions between transverse and longitudinal waves.
The difference between transverse and longitudinal waves is that they revolve around wave motion, specifically, in the way that they oscillate.
When a wave induces material movement that is perpendicular to the direction of energy transfer, we categorise it as a transverse wave.
On the other hand, if the movement happens in a parallel direction of transferred energy, we label the wave longitudinal.
To ensure clarity, let's break down each of these distinctions one at a time.
A Definition and Examples of Transverse Waves
When it comes to the difference between transverse and longitudinal waves, a good way to describe it, is to visualise it.
If there were such a thing as a ‘classic’ wave, it would be the transverse wave. These are the familiar-looking waves that tend to appear in diagrams. Transverse waves are the easiest waves to visualise – as they demonstrate a polarisation 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 the displacement of the medium is at right angles to the direction of energy transmission. As you imagine the skipping rope again, notice that the visible wave tends to travel ‘up and down’ along the length of the rope.
Peaks and Troughs
We call the highest point of a transverse wave – which is that moment of maximum displacement of the medium – the ‘peak’. The lowest point, on the other hand, is called a ‘trough’.
As a consequence, it is easy to measure wave amplitude and wavelength in a transverse wave because you can see the distance between the peaks (which is where wave oscillation or wavelength) is measured.
Both of these – wavelength and amplitude – can tell us about the amount of energy being transferred in the wave.
Further 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.
In addition, electromagnetic waves such as light and radio waves are also transverse. In their disturbance of the magnetic field, they polarise alternately – which means that when the electric wave is peaking, the magnetic aspect is in a trough.
What are Longitudinal Waves?
In a transverse wave, the medium's polarisation results from the displacement. In contrast, a longitudinal wave experiences this disturbance in a manner that doesn't create perpendicular peaks and troughs.
In a longitudinal wave, the displacement aligns with the same direction of the energy travel. Instead of visualising a skipping rope, envision a slinky – a lengthy spiral or a spring.
If you were to lay a slinky flat on a table and push one end, what would occur? The spring's segments would compress and expand sequentially. Unlike the sideways oscillation, the energy would propagate within the structure of the slinky itself.
These motions are referred to by scientists as compressions and rarefactions, and represent the longitudinal counterparts of the transverse peaks and troughs.
Compressions denote regions in the medium where particles, or the coils of the slinky, are closely packed. In these areas, the pressure is significantly high, allowing the medium to push itself apart again. On the other hand, rarefactions represent low-pressure zones where the particles of the medium are more spaced out.
When measuring the amplitude, frequency, or wavelength of a longitudinal wave, the points of highest compression serve as the reference points.
Longitudinal Waves Examples
Beyond the slinky, which incidentally can also support transverse waves, there are numerous longitudinal waves examples too.
An easy-to-understand illustration of this is sound waves. While they propagate effectively through air, they travel at an even faster pace through liquids and solids. This is because these denser mediums allow for greater compressions of their particles.
Can you think about other longitudinal waves examples?
What are Surface Waves?
Now that we have looked at the difference between transverse and longitudinal waves, there is a third category to note.
These are surface waves and the most well-known example of this are ocean waves which occur at the boundary between two different media.
Interestingly, these waves are a blend of both transverse waves and longitudinal waves and have longitudinal and transverse characteristics.
Now, do you want to know more about sound waves?
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