Cambridge IGCSE Physics · Syllabus 0625 · Waves
Longitudinal Wave
What is Longitudinal Wave?
A longitudinal wave is one in which the direction of vibration is parallel to the direction of propagation; sound waves and seismic P-waves can both be modelled as longitudinal.
This definition is part of the Waves chapter in Cambridge IGCSE Physics.
Longitudinal Wave in context
Sound is produced by a vibrating source and travels as a longitudinal wave: the particles of the medium oscillate back and forth parallel to the direction the wave travels. Because the disturbance is handed on from particle to particle, sound cannot travel through a vacuum. Its frequency sets the pitch and its amplitude sets the loudness. Humans hear roughly 20 Hz to 20 000 Hz; above 20 kHz the sound is ultrasound.
Common mistakes with Longitudinal Wave
- 3. “Particles in a longitudinal wave follow a sinusoidal path.” Core 3.1.6 Why wrongThe smooth curve drawn for a longitudinal wave is a graph of pressure or displacement against position, not a picture of a particle’s trajectory. CorrectEach particle moves back and forth along a straight line, parallel to the direction of energy transfer. Say this“In a longitudinal wave the particles vibrate parallel to the direction of energy transfer; the curve is a graph, not a path.” CheckSound travels east. State the direction in which an air molecule oscillates.
Examiner tips on Longitudinal Wave
- Answering “state the difference”. A difference needs both halves and a stated reference direction. “Transverse vibrates up and down” states nothing on its own, because up and down is meaningless without saying what it is relative to. Write: “In a transverse wave the vibration is at right angles to the direction of propagation; in a longitudinal wave the vibration is parallel to it.” The syllabus wording is direction of propagation, so use that phrase rather than “direction of energy transfer” when the question asks for the definition.