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A waveguide is an object that directs the flow of any type of wave. Some of the most basic types include wires or hollow pipes that can guide sound waves. Often, waveguides are used to transfer electromagnetic waves between locations, typically using a hollow tube made of electrically conductive metal. Waveguides are commonly used to transfer power or communication signals. Each type of wave requires a different type of guide.
The type of wave that is being guided dictates the waveguide's geometry. The guide's width must, in most cases, be on the same order of magnitude as the length of the waves it will guide. This means that an optical fiber that guides high-frequency light waves will not be effective in guiding low-frequency sound waves. Other factors, like how much loss of signal or power is acceptable, also help determine the best waveguide for a specific task.
Electromagnetic waves in space normally radiate out in all directions from their point of origin. This causes the radiation to lose power in proportion to the square of its distance from that point of origin. A waveguide allows the radiation to propagate in only one dimension under ideal circumstances, preventing it from losing power as it propagates.
Waveguides work by reflecting the wave off the walls of the guide. Ideally, the waves will propagate in a zigzag pattern within the waveguide. This means that a waveguide often works best when it has a circular or square cross-section.
There are many different types of waveguides, from optical fibers that allow for the transmission of data via light waves to the Sound Frequency and Ranging (SOFAR) channel, a layer of water in the ocean that acts as a natural waveguide for whale song. Radar systems use waveguides to direct radio waves to an antenna so that they may be transmitted at the proper impedance. Scientific instruments also use waveguides to measure the acoustic and optical properties of various objects.
Waveguides have been used for centuries, long before anyone understood how they worked. The first waveguide designed for sound waves was proposed by J.J. Thomson, a British physicist, in 1893, and tested by another physicist, O.J. Lodge, the next year. Work continued throughout the late 19th and early 20th century, with scientists studying the use of optical fibers as waveguides for visible light in the 1920s.