We are all familiar with waves in the ocean, but did you know there are different kinds of waves? One such type of wave is called a longitudinal wave. We will explore what this means and some properties of longitudinal waves in this article.
What are Longitudinal Waves?
First, note that waves are ways that energy can travel without matter having to go along with it. A sound wave pushes the air in front of it with information, transferring vibrational kinetic energy through the air. So, what is a longitudinal wave?
Longitudinal waves are a specific type of wave where the disturbed particles oscillate in the same direction that the wave propagates.
The material that a wave travels through is called a medium. This can be water in the case of ocean waves, or the Earth itself in the case of seismic waves. The particles within this medium must move back and forth for the wave to propagate or traverse forward. This back and forth motion can be in any direction; however, for longitudinal waves, the particles in the medium can only move parallel to the direction that the wave is traveling.
Longitudinal waves have compressions, a period in the wave when two neighboring particles are pushed close together, and rarefactions, a period where two neighbor particles are pulled apart. These are analogous to crests and troughs in transverse waves. Finally, only transverse waves are capable of being polarized, as well as aligned. This can’t be done with a longitudinal wave. This works because, as the wave travels, each particle in the medium is slightly out of phase. When one particle begins to move in this back and forth motion, the next particle in the line is delayed slightly before starting the same motion. With all particles in the medium acting like this, the wave can propagate forward towards the direction it is heading.
Transverse vs Longitudinal Waves
A transverse wave is incredibly similar to a longitudinal wave, only differing in a single key aspect. Where longitudinal waves have the particles in the medium propagating parallel to the wave’s direction, a transverse wave will instead have the particles in the medium traveling perpendicularly to the wave’s direction.
Due to this singular but, big difference, these waves will behave very differently. Due to the nature of the propagation, a transverse wave cannot travel through a medium that is a gas, only a solid or a liquid. This is not a problem for a longitudinal wave, as they can propagate through any kind of medium, be it solid, liquid or gas.
Another consequence of this difference in motion is that transverse waves are two-dimensional and longitudinal waves are one-dimensional.
An example of how a transverse wave oscillates.
A longitudinal wave is still a wave, meaning that it takes information from one location to another. This information is in the out of phase motion of the particles as the wave propagates, known as compressions and rarefactions.
A Longitudinal Wave Diagram
Below is a diagram showing the key features of a longitudinal wave:
An example of how a longitudinal wave oscillates. Note the compressions and rarefactions.
What are some Examples of Longitudinal Waves?
Longitudinal waves exist everywhere in our everyday life, and you just have to look in any direction to find a good example.
Longitudinal waves: Sound waves
We have already mentioned one example of a longitudinal wave we see every day, or should I say hear every day. Sound waves are longitudinal waves, which we should know anyway due to sound traveling through the air, and we already know that only longitudinal waves can do that. When a sound is made, the source of that sound is hitting the air right in front of that source many times a second, and this hitting pushes a longitudinal wave forward, right into our ears for us to hear.
Next time you’re near a speaker, try placing your hand in front of the source of the sound on it. You’ll be able to feel something pushing at your hand, these are the longitudinal sound waves! You can feel them pushing as they move forwards and backward instead of up and down.
The center of the speaker will be the source of the sound, putting your hand over it will allow you to feel the vibrational waves pushing forward.Wikimedia Commons
Another longitudinal wave can be caused directly by sound waves as well. If an object capable of vibration hits a sound wave of a particular frequency, it will begin to vibrate at that frequency. This vibration acts as a longitudinal wave throughout the object. For example, when a glass is shattered through a high frequency passing through it. If the sound wave is hitting the glass with a high enough amplitude and is propagating at the right frequency for the glass to vibrate too, the glass will begin to vibrate so aggressively that it could eventually shatter! This frequency is called the resonant frequency, and every material has one, in which if a wave of this frequency passes through it, it will cause the material to oscillate at that same frequency in increasing amplitude until this frequency causes the material to degrade.
Longitudinal Waves: Earthquakes
You may have been thinking that longitudinal waves only travel through the air, as we’ve not seen any that can travel in a liquid or solid yet. However, a particularly dangerous kind of longitudinal wave that travels through the ground is an earthquake. Earthquakes aren’t just longitudinal, they are made up of different waves, and the longitudinal wave in an earthquake is known as a P wave. These are the waves that come before the big and particularly dangerous S waves which are transverse, and they typically don’t cause a lot of damage, but it is still possible to feel them. These P waves come first due to the fact that longitudinal waves travel faster than transverse waves. They will only ever travel on the ground, this back and forth motion that goes hand in hand with longitudinal waves is why the ground will move left to right when these P waves are active.
The reason that animals can detect earthquakes before us is due to these P waves! They are rarely noticed by us before the larger S waves, but the heightened senses of many different animals such as dogs and cats can register these waves and seek shelter before any real damage can take place.
A surprising but also dangerous kind of real longitudinal wave will occur in large tidal waves. Since these kinds of waves are known for going up and down, you would be forgiven for thinking that they are entirely transverse waves, but this isn’t the case. Eventually, the waves shrink down, and the water that the wave is traveling through starts moving in parallel with the direction of the wave.
Longitudinal Wave - Key takeaways
A longitudinal wave is a wave where the particles of the medium it is traveling in move in parallel with the direction of travel.
The wave moves forward due to the particles of the medium being slightly out of phase with each other.
Transverse waves are similar to longitudinal waves, however, they move the particles in their medium perpendicular to the direction of travel instead of in parallel.
Transverse waves differ more so due to this, by existing in two dimensions, having troughs and crests, and not being able to travel through gasses.
There are many real-world examples of longitudinal waves, including some natural disasters like earthquakes and tidal waves, to the sound waves that we create and listen to every day.
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