Mechanical waves can be classified into two categories based on the motion of the particles – longitudinal and transverse. In longitudinal waves, the vibration or periodic disturbance occurs in the same direction as the wave’s propagation. In transverse waves, the vibration or periodic disturbance occurs in a direction perpendicular to the wave’s propagation.
A. Longitudinal Wave
When a longitudinal wave propagates, it compresses and rarifies the medium. The particles in the medium move back and forth along the direction of propagation of the wave. As a result, there are regions with high and low pressures. A few examples of longitudinal waves are the sound wave, the ultrasound wave, and the seismic P-wave.
B. Transverse Wave
A sine or a cosine function usually represents a transverse wave, called a wave function. The displacement of a wave from its axis of propagation is proportional to the sine or cosine of the angle the displacement vector makes with the axis. Examples of transverse waves include electromagnetic waves (light and heat), ripples on the water surface, vibrations on a guitar string, and seismic S-wave.
A. Body Wave
As the name suggests, body waves travel through the body of the Earth’s interior and have a frequency higher than the surface wave. Body wave is divided into P or primary and S or secondary waves. P is longitudinal, and S is transverse.
B. Surface Wave
A surface wave is similar to a transverse wave but travels along the boundary between the Earth’s surface and air, i.e., through the crust. It has a lower frequency than body waves and is responsible for the damage and destruction caused by earthquakes. The particles in a surface wave move in a circular or elliptical motion that diminishes at a certain depth from the surface. Two common types of surface waves are the Raleigh wave and the Love wave.
