Waveguide

A waveguide is a structure that makes waves move in only one direction. Common types of waveguides include acoustic waveguides which direct the movement of sound and optical waveguides which direct light. Radio-frequency waveguides direct electromagnetic waves other than light. An example of these waves are radio waves that are used by RADAR systems.

Without a waveguide, waves would move in three dimensions. As they did this, they would get weaker according to the inverse square law.

There are different types of waveguides for different types of waves. The most common meaning is a hollow metal pipe used to carry high frequency radio waves. These are usually microwaves. Transparent dielectric waveguides and optical fibers are used as waveguides for light.   In acoustics, air ducts and horns are used as waveguides for sound. They are used in musical instruments and loudspeakers. Metal rods with special shapes are used to conduct ultrasonic waves in ultrasonic machining.

The frequency of the wave also affects the size of a waveguide. Each waveguide has a cutoff wavelength that is determined by its size. It can not be used for waves that have a greater wavelength than its cutoff value. For example, an optical fiber that guides light can not be used for microwaves because they have a much larger wavelength. Some naturally occurring structures can also act as waveguides. The SOFAR channel layer in the ocean can guide the sound of whale song across very large distances. 
Any shape of waveguide can be used with EM waves. Commonly used waveguides are rectangular and circular in shape.

References

Category:Electromagnetic radiation
Category:sound
Category:Optics