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photon

The photon is the fundamental particle of visible light.

In some ways, visible light behaves like a wave phenomenon, but in other respects it acts like a stream of high-speed, submicroscopic particles. Isaac Newton was one of the first scientists to theorize that light consists of particles. Modern physicists have demonstrated that the energy in any electromagnetic field is made up of discrete packets. The term photon (meaning "visible-light particle") was coined for these energy packets. Particle-like behavior is not restricted to the 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 a particular 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 the electromagnetic wavelength. In media other than a vacuum, the speed is reduced. For example, visible light travels more slowly through glass than through outer space. Radio waves travel more slowly through the polyethylene in a transmission line than they do through the atmosphere. The ratio of the speed of the photons in a particular 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 to some extent on the wavelength.

The shorter the wavelength of an electromagnetic disturbance, the more energy each photon contains. In fact, this relationship is so precise that a mathematical formula applies. If e represents the energy (the unit of measurement is the joule) contained in each photon and s represents the electromagnetic wavelength (in meters), then

e = hc / s

where h is Planck's constant (approximately equal to6.626 times 10 -34 joule-second) and c is the speed of electromagnetic-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 the frequency of an electromagnetic field (in hertz), then

e = hf

The energy contained in a single photon does not depend on the intensity of the radiation. At any specific wavelength -- say, the wavelength of light emitted by a helium-neon laser -- every photon contains exactly the same amount of energy, whether the source appears as dim as a candle or as bright as the sun. The brilliance or intensity is a function of the number of photons striking a given surface area per unit time.

See a brief introduction to the history of light:

This was last updated in November 2015

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its great...
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If  photon had mass would it lose speed in a reverse venture?
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If a photo had mass would it lose velocity in a reverse venture? 
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