photonIn some ways, visible light behaves like a wave phenomenon, but in otherrespects it acts like a stream of high-speed, submicroscopic particles. Isaac Newtonwas one of the first scientists to theorize that light consists of particles. Modernphysicists have demonstrated that the energy in any electromagnetic field is made up of discretepackets. The term photon (meaning "visible-light particle") hasbeen coined for these energy packets. Particle-like behavior is not restricted tothe visible-light portion of the electromagnetic radiation spectrum , however. Radio waves, infrared rays, visible light,ultraviolet rays, X rays, and gamma rays all consist of photons, each of which contains aparticular amount of energy that depends on the wavelength . Photons travel through empty space at a speed of approximately 186,282miles (299,792 kilometers) per second. This is true no matter what theelectromagnetic wavelength. In media other than a vacuum, the speed is reduced. For example, visible light travels more slowly through glass than through outerspace. Radio waves travel more slowly through the polyethylene in a transmissionline than they do through the atmosphere. The ratio of the speed of the photons in aparticular medium to their speed in a vacuum is called the velocity factor . This factor is always between 0 and 1 (or 0 and 100 percent), and it depends tosome extent on the wavelength. The shorter the wavelength of an electromagnetic disturbance, the moreenergy each photon contains. In fact, this relationship is so precise that amathematical formula applies. If e represents the energy (the unit of measurement is the joule )contained in each photon and s represents the electromagnetic wavelength (inmeters), then e = hc / s where h is Planck's constant (approximately equal to6.626 times 10 -34 joule-second) and c is the speed ofelectromagnetic-field propagation in the medium in question (approximately 2.998 times 10 8 meters per second in a vacuum). A simpler formula applies to frequency . If f represents thefrequency of an electromagnetic field (in hertz ),then e = hf The energy contained in a single photon does not depend on the intensity of theradiation. At any specific wavelength -- say, the wavelength of light emitted by ahelium-neon laser -- every photon contains exactly the same amount of energy, whether thesource appears as dim as a candle or as bright as the sun. The brilliance orintensity is a function of the number of photons striking a given surface area per unittime.
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| Last updated on:
Dec 19, 2006 |
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