Longitudinal waves are a type of wave where the particles of the medium move parallel to the direction of the wave's travel. This means that as the wave moves, it compresses and stretches the medium, creating areas of high pressure (compressions) and low pressure (rarefactions). Sound waves in air are a common example of longitudinal waves, as the air molecules vibrate back and forth along the same direction that the sound is moving. In contrast to transverse waves, where particles move perpendicular to the wave direction, longitudinal waves are essential in various fields, including acoustics and engineering. Understanding how longitudinal waves work helps in applications like sonar technology and ultrasound imaging, where sound waves are used to detect objects or create images.