Tuesday, February 14, 2012


Light Amplification by Stimulated Emission of Photons 
can be used, for example, to burn a DVD

Max Planck said that energy - whatever it is! - is packed in small packages he called quanta. At those times physicists coined words in the real of electromagnetism that have since become everyday vocabulary, photon, electron, proton.

Photon is the elementary particle of light.

The subject is huge even photons are so small. In order to save your click time I copy here the introduction part of the detailed wikipedia article on photons. As a fellow student I give you the quoted text already partly processed by highlighting the contents by adding paragraphs and emphasizing words using italics or bold instead of the usual yellow or pink marker pen. The wikipedia text itself has not been otherwise modified.

In physics, a photon is an elementary particle,
  • the quantum of light and all other forms of electromagnetic radiation, and 
  • the force carrier for the electromagnetic force. 

The effects of this force are easily observable at both the microscopic and macroscopic level, because the photon has no rest mass; this allows for interactions at long distances.

Like all elementary particles, photons are currently best explained by

  • quantum mechanics and exhibit 
  • wave–particle duality, exhibiting properties of both waves and particles. 

For example, a single photon may be
  • refracted by a lens or exhibit wave interference with itself, but also 
  • act as a particle giving a definite result when its position is measured. 

The modern concept of the photon was developed gradually by Albert Einstein to explain experimental observations that did not fit the classical wave model of light.

In particular, the photon model accounted for
  • the frequency dependence of light's energy, and explained 
  • the ability of matter and radiation to be in thermal equilibrium. 

It also accounted for anomalous observations, including the properties of black body radiation, that other physicists, most notably Max Planck, had sought to explain using semiclassical models, in which light is still described by Maxwell's equations, but the material objects that emit and absorb light are quantized.

Although these semiclassical models contributed to the development of quantum mechanics, further experiments validated Einstein's hypothesis that light itself is quantized; the quanta of light are photons.

In the Standard Model of particle physics, photons are described as a necessary consequence of physical laws having a certain symmetry at every point in spacetime. The intrinsic properties of photons, such as charge, mass and spin, are determined by the properties of this gauge symmetry.

The photon concept has led to momentous advances in experimental and theoretical physics, such as

  • lasers
  • Bose–Einstein condensation
  • quantum field theory, and 
  • the probabilistic interpretation of quantum mechanics

It has been applied to photochemistry, high-resolution microscopy, and measurements of molecular distances. Recently, photons have been studied as elements of quantum computers and for sophisticated applications in optical communication such as quantum cryptography.

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