 The wave guide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting expansion to one dimension or two. There is a similar effect in water waves constrained within a kind ale, or guns that have barrels which restrict hot gas expansion to maximize energy transfer to their bullets. Without the physical constraint of the wave guide, wave amplitudes decrease according to the inverse square law as they expand into three dimensional space. There are different types of wave guides for each type of wave. The original and most common meaning is a hollow conductive metal pipe used to carry high frequency radio waves, particularly microwaves. The geometry of a wave guide reflects its function. Slab wave guides confine energy in one dimension, fiber or channel wave guides into dimensions. The frequency of the transmitted waves also dictates the shape of a wave guide, an optical fiber guiding high frequency light will not guide microwaves of a much lower frequency. Some naturally occurring structures can also act as wave guides. The so far channel layer in the ocean can guide the sound of whale song across enormous distances. Waves propagate in all directions in open space as spherical waves. The power of the wave falls with the distance r from the source as the square of the distance inverse square law. A wave guide confines the wave to propagate in one dimension, so that, under ideal conditions, the wave loses no power while propagating. Due to total reflection at the walls, waves are confined to the interior of the wave guide.