Sep 2020

Theory Of Wave Motion: Longitudinal And Transverse Waves

September 30, 2020

Waves exist everywhere around us, but how can we describe them in a way that enhances our understanding of them? Let’s start by explaining what wave motion is.

Wave motion

Wave motion is defined as the transfer of energy without involving the movement of matter. Instead, the particles of a medium simply vibrate about their fixed position. There are two examples to help visualise wave motion:

1. Vibrations in ropes: Hold on to one end of a rope, with the other end tied to a stationary object. By moving your end of the rope up and down, you’re generating waves which travel all the way to the other end. In other words, you’re transferring energy to the other end of the rope without transferring matter.

2. Waves in a ripple tank: When waves progress along a ripple tank, the water particles move up and down, producing transverse waves. Although the waves appear to ripple in circular motions, the water particles are actually moving perpendicularly.

Wave terms

When studying waves and their interactions with other objects, we need ways to quantify various aspects of waves. With that, let’s first establish the definition and SI Units of the main terms in this topic.

TermDefinitionSI Unit
Velocity (m)The distance a wave travels per secondm/s
Frequency (f)The number of crests (peaks) or troughs that pass a point per second; the number of complete waves generated per second.Hz (hertz)
Wavelength(λ)The distance between two successive crests or two successive troughs in a wavem (metre)
AmplitudeThe magnitude of the maximum displacement from the rest position (or equilibrium) of the wave. This is based on the height of the crest or depth of trough from the rest position.m (metre)
WavefrontA wavefront is defined as an imaginary line that connects all the crests (or points in phase) on a wave. It can be represented by straight lines, concentric centres, and other shapes. 

The relationship of the velocity, frequency, and wavelength of a wave can be described by the following formula:

  • Velocity (v) = Frequency (f) x Wavelength (λ)

Longitudinal and transverse waves

Waves come in different forms, namely, longitudinal and transverse waves. Let’s take a look at how they differ!

Waves that travel in a direction parallel to the direction of vibrations are referred to as longitudinal waves. Some examples of longitudinal waves include sound waves and ultrasound waves. On the other hand, transverse waves are waves which travel in a direction perpendicular to the direction of vibrations, such as ripples made on the surface of water and electromagnetic waves.

With that, we’ve identified one main difference between longitudinal and transverse waves: the direction in which they travel! Now, let’s explore other factors that set each of them apart from one another, through the table below.

Difference between longitudinal and transverse waves
Longitudinal WaveTransverse Wave
The displacement of particles is parallel to the direction of the wave.The displacement of particles is perpendicular to the direction of the wave.
It acts in one dimension.It acts in two dimensions.
The wave cannot be polarised or aligned.The wave can be polarised or aligned.
The wave can be produced regardless of the medium (gas, liquid and sound).The wave can only be produced in solid and on the surface of a liquid.
It consists of rarefactions and compressions.It is made of troughs and crests.

Conclusion

The basic concepts of waves may seem simple, but the tough part is when you need to use them to solve problems and link them to other topics! If you find yourself needing more help with this or other topics, don’t be shy to look for help.

With the help of a Physics tuition teacher, you can clarify all your misunderstandings while immersing yourself in more Physics learning. Eventually, you’ll be able to develop your critical thinking skills while enhancing your learning experience!

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