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Electromagnetic Radiation, waves produced by the oscillation
or acceleration of an electric charge. Electromagnetic waves have both electric and magnetic components. Electromagnetic radiation can be arranged in a spectrum that extends from waves of extremely high frequency and short wavelength to extremely low frequency and long wavelength. Visible light is only a small part of the electromagnetic spectrum. In order of decreasing frequency, the electromagnetic spectrum consists of gamma rays, hard and soft X-rays, ultraviolet radiation, visible light, infrared radiation, microwaves, and radio waves.
Properties
Electromagnetic waves need no material medium for their transmission. Thus, light and radio waves can travel through interplanetary and interstellar space from the Sun and stars to the Earth. Regardless of their frequency and wavelength, electromagnetic waves travel at the same speed in a vacuum. The value of the metre has been defined so that the speed of light is exactly 299,792.458 km (approximately 186,282 mi) per second in a vacuum. All the components of the electromagnetic spectrum also show the typical properties of wave motion, including diffraction and interference. The wavelengths range from billionths of a centimetre to many kilometres. The wavelength and frequency of electromagnetic waves are important in determining their heating effect, visibility, penetration, and other characteristics.
Theory
The British physicist James Clerk Maxwell laid out the theory of electromagnetism in a series of papers published in the 1860s. He deduced that electromagnetic waves must exist and stated that visible light consisted of such waves.
Physicists had known since the early 19th century that light travels as a transverse wave (a wave in which the vibrations move in a direction perpendicular to the direction of the advancing wave front). They assumed, however, that the wave required some material medium for its transmission, so they thought that there was an extremely diffuse substance, called ether, which was the unobservable medium. Maxwell's theory made such an assumption unnecessary, but the ether concept was not abandoned immediately, because it fitted in with the Newtonian concept of an absolute space-time frame for the universe. A famous experiment conducted by the American physicist Albert Abraham Michelson and the American chemist Edward Williams Morley in the late 19th century undermined the ether concept and was important in the development of the theory of relativity. This work led to the realization that the speed of electromagnetic radiation in a vacuum is the same, regardless of the velocity of the source or the observer.
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