1. What Longitudinal Waves Are
Longitudinal waves are waves in which the particles of the medium move back and forth along the same direction in which the wave itself travels. Instead of forming crests and troughs like transverse waves, these waves create compressions and rarefactions.
I usually picture a slinky spring being pushed and pulled along its length — bunching up in some regions and spreading out in others. That is exactly how a longitudinal wave behaves.
2. Definition of Longitudinal Waves
Definition: Longitudinal waves are waves in which particles of the medium vibrate parallel to the direction of wave propagation.
This parallel motion of particles produces alternating regions of high and low pressure.
3. Key Features of Longitudinal Waves
These waves show two distinct regions that repeat along the length of the medium:
- Compressions: particles are close together; pressure is high.
- Rarefactions: particles are spread apart; pressure is low.
- Wavelength: distance between two consecutive compressions or two consecutive rarefactions.
These repeating patterns move forward, carrying energy along the medium.
3.1. Compressions and Rarefactions
In compressions, the medium is squeezed; in rarefactions, it is stretched. As the wave moves, these regions travel, but the particles themselves only oscillate back and forth around their positions.
3.2. Visual Picture
If you push and pull a slinky rapidly along its length, you’ll see certain parts bunch up (compressions) and others spread out (rarefactions). These regions travel along the slinky even though the individual coils merely move slightly back and forth.
4. How Particles Move in Longitudinal Waves
Each particle in the medium oscillates in the same direction as the wave travels. The particles do not move forward with the wave — instead, they simply move back and forth around their mean positions.
4.1. Example: Sound Wave in Air
When someone speaks, their vocal cords create disturbances in air pressure. Air particles compress in some areas and spread out in others. These alternating pressure variations travel through the air as a longitudinal wave and eventually reach our ears.
5. Examples of Longitudinal Waves
Most everyday sounds and vibrations are longitudinal waves. Common examples include:
- Sound waves in air or any medium.
- Compression waves in springs or slinkies.
- Ultrasonic waves used in medical imaging.
- Seismic P-waves that travel inside the Earth.
These waves rely on particles compressing and expanding along the direction of motion.
6. Wavelength and Frequency in Longitudinal Waves
Just like in transverse waves, wavelength and frequency describe the nature of the disturbance:
- Wavelength: distance between successive compressions or rarefactions.
- Frequency: number of waves passing a point per second.
Wave speed is given by the simple formula:
\( v = f\lambda \)
7. Why Longitudinal Waves Matter
Understanding longitudinal waves is key to studying sound, vibrations, and many natural phenomena. These waves explain how we hear, how machines transmit vibrations, and how energy travels through gases, liquids, and even solids.
Once the idea of compressions and rarefactions becomes clear, the behaviour of sound and many mechanical waves becomes very easy to understand.