Growth Hormone +Photns

Topics: Growth hormone, Photon, Photoelectric effect Pages: 22 (7012 words) Published: September 5, 2010
The photoelectric effect is a phenomenon in which electrons are emitted from matter (metals and non-metallic solids, liquids or gases) as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as "photoelectrons".[1][2] As it was first observed by Heinrich Hertz in 1887,[2] the phenomenon is also known as the "Hertz effect",[3][4] although the latter term has fallen out of general use. Hertz observed and then showed that electrodes illuminated with ultraviolet light create electric sparks more easily.[citation needed] The photoelectric effect takes place with photons with energies from about a few electronvolts to, in high atomic number elements, over 1 MeV. At the high photon energies comparable to the electron rest energy of 511 keV, Compton scattering, another process, may take place, and above twice this (1.022 MeV) pair production may take place.[5] Study of the photoelectric effect led to important steps in understanding the quantum nature of light and electrons and influenced the formation of the concept of wave–particle duality.[1] The term may also, but incorrectly, refer to related phenomena such as the photoconductive effect (also known as photoconductivity or photoresistivity), the photovoltaic effect, or the photoelectrochemical effect which are, in fact, distinctly different.[citation needed] Contents

• 1 Introduction and early historical view
• 2 Modern view
• 3 Traditional explanation
○ 3.1 Experimental results of the photoelectric emission
○ 3.2 Mathematical description
○ 3.3 Three-step model
• 4 History
○ 4.1 Early observations
○ 4.2 Hertz's spark gaps
○ 4.3 Stoletov: the first law of photoeffect
○ 4.4 JJ Thomson: electrons
○ 4.5 Radiant energy
○ 4.6 Von Lenard's observations
○ 4.7 Einstein: light quanta
○ 4.8 Effect on wave–particle question
• 5 Uses and effects
○ 5.1 Photodiodes and phototransistors
○ 5.2 Photomultipliers
○ 5.3 Image sensors
○ 5.4 The gold-leaf electroscope
○ 5.5 Photoelectron spectroscopy
○ 5.6 Spacecraft
○ 5.7 Moon dust
○ 5.8 Night vision devices
• 6 Cross section
• 7 See also
• 8 References
• 9 External links
[edit] Introduction and early historical view
When a surface is exposed to electromagnetic radiation above a certain threshold frequency (typically visible light for alkali metals, near ultraviolet for other metals, and extreme ultraviolet for non-metals), the radiation is absorbed and electrons are emitted. This phenomenon was first observed by Heinrich Hertz in 1887. Johann Elster (1854-1920) and Hans Geistel (1855-1923), students in Heidelberg, developed the first practical photoelectric cells that could be used to measure the intensity of light[6]. In 1902, Philipp Eduard Anton von Lenard observed that the energy of individual emitted electrons increased with the frequency (which is related to the color) of the light. This appeared to be at odds with James Clerk Maxwell's wave theory of light, which was thought to predict that the electron energy would be proportional to the intensity of the radiation. In 1905, Albert Einstein solved this apparent paradox by describing light as composed of discrete quanta, now called photons, rather than continuous waves. Based upon Max Planck's theory of black-body radiation, Einstein theorized that the energy in each quantum of light was equal to the frequency multiplied by a constant, later called Planck's constant. A photon above a threshold frequency has the required energy to eject a single electron, creating the observed effect. This discovery led to the quantum revolution in physics and earned Einstein the Nobel Prize in Physics in 1921.[7] [edit] Modern view

It has been shown that it is not necessary for light to be "quantized" to explain the photoelectric effect[8]. The most common method...
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