The longitudinal mode of a waveguide is a particular standing wave pattern formed by waves confined in the cavity.

This is good for longitudinal modes of vibration.

A common example of longitudinal modes are the light wavelengths produced by a laser.

The frequency spacing of longitudinal modes in the cavity is then given by:

These standing waves form a discrete set of frequencies, known as the longitudinal modes of the cavity.

When more than one longitudinal mode is excited, the laser is said to be in "multi-mode" operation.

When only one longitudinal mode is excited, the laser is said to be in "single-mode" operation.

These are often associated with longitudinal modes in the acoustic range.

Lattice dynamics have been investigated for longitudinal as well as transverse modes.

The third index describes the longitudinal mode caused by the interference pattern of the forward travelling and reflected waves.