Electromagnetic Radiation light is that form of electromagnetic radiation that allows the eye to see
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Electromagnetic Radiation LIGHT is that form of electromagnetic radiation that allows the eye to see. In fundamental theory, ELECTROMAGNETIC RADIATION is energy radiated in the form of a wave caused by an electric field interacting with a magnetic field. It is the result of the acceleration of charged particles. Contemporary theory suggests that electromagnetic radiation also behaves as a group of particles called photons. Nevertheless, light tends to travel in a straight-line path unless influenced by a gravitational, magnetic, or some other force. Unimpeded, light travels at the speed of 186 000 miles per second (300 000 000 meters per second) in air or in a vacuum. Categorization of Electromagnetic Radiation 1. Wavelength is measured as the distance from one peak of one wave to the next wave. It is expressed in meters or nanometers (1 nm = 0.000 000 001 meter or one billionth of a meter). One inch contains about 25.4 million nanometers. 2. Frequency is the number of wave cycles per second. It is expressed in units of hertz (Hz). TV and radio waves are several meters long with frequencies ranging from 10 kilohertz to 300 000 megahertz. In contrast, visible light waves are only about 0.000 000 5 m in length Wavelength and frequency of electromagnetic radiation are dependent on the source emitting the radiation. Large antennas such as those found on the tops of buildings and mountains must be used to generate radio and TV waves. Gamma radiation, the shortest and most powerful form of electromagnetic radiation, result from changes within the nucleus of the atom Telecommunication Systems Historical Perspective Methods of communicating over long distances have evolved over many millennia. Although carrier pigeons were used to convey messages from about 700 B.C.E., the first long-distance communication systems were based on signals of sound and light (e.g. drums and horns, smoke signals and beacon fires). Signal fires alerted the British of the arrival of the Spanish Armada in 1588 C.E. The Chinese used rockets as signals to warn of an imminent attack on the Great Wall. Native Americans communicated by covering and uncovering a bonfire with a blanket to produce smoke signals or by beating drums. The British Navy sent signals at night by raising and lowering a lantern, which coincidentally was the same way Paul Revere was signaled with news of the arrival of the British. In instances when clear vision was difficult (e.g., fog), bells or whistles and fired weapons sent signals. Until almost 1800, traditional long-distance communication was by horse-mounted dispatch riders. 1793 Frenchman Claude Chappe developed an optical telegraph (semaphore) system of stations built on rooftops or towers that were visible from a great distance. Each semaphore station consisted of a column-like tower with a moveable beam. Attached to the beam were two moveable arms. The beam and arms were swiveled with ropes, conveying different signal patterns representing upper- and lowercase letters, punctuation marks, and numbers. A set of patterns was translated into words by an observer at another station, who then sent it on to the next station. This system allowed the French to send a concise message over 100 miles (160 km) in less than 5 min as long as visibility was good. Swede A. N. Edelcrantz developed another type of optical telegraph system with ten collapsible iron shutters, which when placed in various positions formed combinations of numbers that were translated into letters, words, or phrases. Crude semaphore systems were also used in Boston, New York City, and San Francisco at that time. Download 13.98 Kb. Share with your friends:
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