What Is a Longitudinal Wave? (Grade 10 NSC Matric Physical Sciences): Revision Notes
What Is a Longitudinal Wave?
Introduction and key concepts
We have previously studied transverse pulses and waves. Now we explore a different type of wave called a longitudinal wave.
The key difference lies in how particles move:
- In transverse waves: particles move perpendicular (at right angles) to the wave's direction
- In longitudinal waves: particles move parallel (in the same direction) as the wave's motion
Common Examples:
- Transverse waves: water waves, waves on a string
- Longitudinal waves: sound waves, compression waves in springs
Definition of longitudinal waves
Understanding longitudinal waves requires us to examine how particles behave within the medium as the wave passes through.
DEFINITION: Longitudinal waves
A longitudinal wave is a wave where the particles in the medium move parallel to the direction of propagation of the wave.
When studying any wave, we examine two types of motion:
- The motion of the particles in the medium
- The motion of the wave itself
Investigation: exploring longitudinal waves with a slinky spring
The best way to understand longitudinal waves is through a practical investigation using a slinky spring.
Practical Investigation: Slinky Spring Demonstration
Method:
- Place a slinky spring on a table
- Hold one end firmly and pull the free end
- Flick the free end back and forth once in the direction of the spring
- Observe what happens and note the direction of the disturbance
Adding a marker:
- Tie a ribbon to the middle of the spring
- Watch carefully what happens to the ribbon when you flick the end
- Describe the motion of the ribbon
- Flick the spring continuously to create a train of pulses (a longitudinal wave)
Key Observations:
- The disturbance moves parallel to the direction in which the spring was pulled
- The ribbon represents one particle in the medium
- The particles in the medium move in the same direction as the wave
- The motion of particles in the spring is back and forth
Key Insight: In longitudinal waves, the direction of particle movement matches the direction of wave propagation.
Compression and rarefaction
Longitudinal waves have different characteristics compared to transverse waves, which is essential to understand for identifying wave types.
- Transverse waves have crests and troughs
- Longitudinal waves have compressions and rarefactions
Definitions:
Compressions: Areas where particles are pushed closer together
Rarefactions: Areas where particles are spread further apart
This creates a pattern of alternating compressed and stretched regions that travel through the medium.
Properties of longitudinal waves
Despite their different particle motion, longitudinal waves share fundamental properties with transverse waves.
Longitudinal waves have the same measurable properties as transverse waves:
- Wavelength
- Amplitude
- Period
- Frequency
- Wave speed
These properties can be defined and measured for longitudinal waves just as they are for transverse waves.
Key Points to Remember:
- Longitudinal waves have particles that move parallel to the wave's direction of travel
- Sound waves are the most common example of longitudinal waves
- A slinky spring perfectly demonstrates how longitudinal waves work
- Instead of crests and troughs, longitudinal waves have compressions and rarefactions
- Longitudinal waves have the same properties as transverse waves (wavelength, frequency, etc.)