Longitudinal and Transverse Waves

Waves transfer energy from place to place without transferring matter. A useful way to sort waves is by how the particles of the medium vibrate compared to the direction the wave travels.

Key wave ideas

• Amplitude: how far particles move from their rest position; linked to energy (bigger amplitude means more energy).
• Wavelength (λ): the distance between matching points, like crest to crest or compression to compression.
• Frequency (f): how many waves pass a point each second (measured in hertz, Hz).
• Wave speed (v): how fast the wave pattern moves. The wave equation is $v = f\lambda$.

Transverse waves

In a transverse wave, vibrations are at right angles to the direction the wave travels. Imagine flicking a rope up and down: the rope moves up and down, but the wave travels along the rope. Peaks are called crests and lows are called troughs. Examples: water waves on the surface, electromagnetic waves (light, radio, microwaves), and seismic S-waves.

Longitudinal waves

In a longitudinal wave, vibrations are parallel to the direction the wave travels. Think of pushing and pulling a stretched slinky along its length: sections squash together (compressions) and spread out (rarefactions) moving along the slinky. Examples: sound in air and seismic P-waves.

Comparing the two

• Direction of vibration: transverse = at right angles; longitudinal = along the same line.
• Wave features: transverse has crests and troughs; longitudinal has compressions and rarefactions.
• Media: sound (longitudinal) needs a medium; electromagnetic waves (transverse) can travel through a vacuum.

Real-world links

• Sound gets louder with bigger amplitude and higher in pitch with higher frequency.
• Water wave height is its amplitude; surfers look for large-amplitude waves.
• Earthquakes send both P-waves (longitudinal) and S-waves (transverse) through the Earth.

Common misconceptions

• Particles do not travel with the wave; they vibrate around a fixed position.
• Louder sound does not mean faster sound; amplitude changes loudness, not speed.
• Shorter wavelength means higher frequency if speed stays the same, because $v = f\lambda$.