Why are electromagnetic waves more useful than matter waves for long-distance communication?
Answers
Answer:
Radio waves are used for wireless transmission of sound messages, or information, for communication, as well as for maritime and aircraft navigation. The information is imposed on the electromagnetic carrier wave as amplitude modulation (AM) or as frequency modulation (FM) or in digital form (pulse modulation). Transmission therefore involves not a single-frequency electromagnetic wave but rather a frequency band whose width is proportional to the information density. The width is about 10,000 Hz for telephone, 20,000 Hz for high-fidelity sound, and five megahertz (MHz = one million hertz) for high-definition television. This width and the decrease in efficiency of generating electromagnetic waves with decreasing frequency sets a lower frequency limit for radio waves near 10,000 Hz.
Answer:
Radio waves
Radio waves are used for wireless transmission of sound messages, or information, for communication, as well as for maritime and aircraft navigation. The information is imposed on the electromagnetic carrier wave as amplitude modulation (AM) or as frequency modulation (FM) or in digital form (pulse modulation). Transmission therefore involves not a single-frequency electromagnetic wave but rather a frequency band whose width is proportional to the information density. The width is about 10,000 Hz for telephone, 20,000 Hz for high-fidelity sound, and five megahertz (MHz = one million hertz) for high-definition television. This width and the decrease in efficiency of generating electromagnetic waves with decreasing frequency sets a lower frequency limit for radio waves near 10,000 Hz.
We cannot hear any electromagnetic radiation. The radiation is used to carry a signal that is converted into a sound wave by the receiver. Medium wavelength radio waves are reflected from the ionosphere, a layer of charged particles in the upper atmosphere, so they can be used for long distance communication.
Because electromagnetic radiation travels in free space in straight lines, late 19th-century scientists questioned the efforts of the Italian physicist and inventor Guglielmo Marconi to develop long-range radio. Earth’s curvature limits the line-of-sight distance from the top of a 100-metre (330-foot) tower to about 30 km (19 miles). Marconi’s unexpected success in transmitting messages over more than 2,000 km (1,200 miles) led to the discovery of the Kennelly-Heaviside layer, more commonly known as the ionosphere. This region is an approximately 300-km- (190-mile-) thick layer starting about 100 km (60 miles) above Earth’s surface in which the atmosphere is partially ionized by ultraviolet light from the Sun, giving rise to enough electrons and ions to affect radio waves. Because of the Sun’s involvement, the height, width, and degree of ionization of the stratified ionosphere vary from day to night and from summer to winter.
Radio waves transmitted by antennas in certain directions are bent or even reflected back to Earth by the ionosphere, as illustrated in Figure 5. They may bounce off Earth and be reflected by the ionosphere repeatedly, making radio transmission around the globe possible. Long-distance communication is further facilitated by the so-called ground wave. This form of electromagnetic wave closely follows Earth’s surface, particularly over water, as a result of the wave’s interaction with the terrestrial surface. The range of the ground wave (up to 1,600 km [1,000 miles]) and the bending and reflection of the sky wave by the ionosphere depend on the frequency of the waves. Under normal ionospheric conditions 40 MHz is the highest-frequency radio wave that can be reflected from the ionosphere. In order to accommodate the large band width of transmitted signals, television frequencies are necessarily higher than 40 MHz. Television transmitters must therefore be placed on high towers or on hilltops.