Know that for a longitudinal wave, the direction of vibration is parallel to the direction of propagation and understand that sound waves and seismic P-waves (primary) can be modelled as longitudinal

Longitudinal Waves

In a longitudinal wave the particles of the medium vibrate parallel to the direction the wave travels. Think of a Slinky being pushed and pulled along its length – the compression and rarefaction move along the same line as the push.

Key examples in physics:

• 🔊 Sound waves – the air molecules compress and expand as the wave moves.
• 🌍 Seismic P‑waves – the first waves that arrive in an earthquake, moving rock particles back and forth along the direction of travel.

Mathematically, the speed of a longitudinal wave in a medium is given by

\$v = f \lambda\$

where \$v\$ is speed, \$f\$ is frequency, and \$\lambda\$ is wavelength.

Exam Tip

When asked to identify the wave type, look for the direction of particle motion:

• If particles move parallel to wave travel, it's a longitudinal wave (e.g., sound).
• If particles move perpendicular, it's a transverse wave (e.g., light).

Use the word “parallel” or “perpendicular” in your answer to show you understand the concept.

Analogy & Example

Picture a line of people holding hands and standing in a straight line. If one person pushes the person next to them forward and then pulls them back, the push and pull travel along the line. Each person moves back and forth along the line – that’s a longitudinal wave. In contrast, if each person lifts their arm up and down while the line moves sideways, that would be a transverse wave.

Key Takeaways

• The direction of particle motion in a longitudinal wave is parallel to the wave’s direction of travel.
• Sound waves and seismic P‑waves are classic examples of longitudinal waves.
• Use the terms parallel and perpendicular to describe wave types in exam answers.