<p>I wanna get stoned and sloshed and wasted and wake up on the other side of April.</p>
<p>should i elaborate…? do we hav a moderator?? i dont wanna be kicked out</p>
<p>me too anu but my mom is pestering me to study for iit and ive been chatting on tit for the last 4 hrs</p>
<p>Really, I think we know what you want to say. No need to elaborate.</p>
<p>Teh modz fear us. Don’t worry about them.</p>
<p>charles: Okay, that was even lamer. And good for you, anurag. I can’t say I don’t know how you feel.</p>
<p>LAWL at anu.</p>
<p>hey yall gonna be back online tonite??? coz its been really fun… i even got to a 100…</p>
<p>I’m going to go learn about the photoelectric effect. Bye.</p>
<p>i hv a question…sry if im getting too personal…but i cant help it…
do girls watch stuff?
sry again if im stepping on personal space
anurag is too big a word to type</p>
<p>Night time’s reserved for bhangra and cricket. :D</p>
<p>You are SO garyish, ‘charlie’. I like you.</p>
<p>Bye ans, I’ll probably be around. I’m not gonna study today if not studying is the last thing I ever do.</p>
<p>Edit: Do girls watch PRON? I don’t know about PRON. Some girls do watch PRON, I’m sure. I can’t say I’ve watched PRON, though, really.</p>
<p>Like I said… Ticket to noobistan</p>
<p>photoelectric effect: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 photoresistivitity), the photovoltaic effect, or the photoelectrochemical effect which are, in fact, distinctly different.[citation needed]
Contents [hide]
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</p>
<p>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. 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.[6]
[edit]Modern view</p>
<p>It has been shown that it is not necessary for light to be “quantized” to explain the photoelectric effect[7]. The most common method employed by physicists to calculate the probability of an atom ejecting an electron relies on “Fermi’s golden rule”. Although based upon quantum mechanics, the method treats the incident light as an electromagnetic wave that causes an atom and its constituent electrons to transition from one energy state (“eigenstate”) to another. It is also important to note that the particle nature of light cannot explain the dependence on polarization with regard to the direction electrons are emitted, a phenomenon that has been considered useful in gathering polarization data from black holes and neutron stars.[8]. Nonetheless, the notion that the photoelectric effect demonstrates the particle nature of light persists in many introductory textbooks.[citation needed]
[edit]Traditional explanation</p>
<p>The photons of a light beam have a characteristic energy determined by the frequency of the light. In the photoemission process, if an electron within some material absorbs the energy of one photon and thus has more energy than the work function (the electron binding energy) of the material, it is ejected. If the photon energy is too low, the electron is unable to escape the material. Increasing the intensity of the light beam increases the number of photons in the light beam, and thus increases the number of electrons emitted, but does not increase the energy that each electron possesses. Thus the energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy of the individual photons. (This is true as long as the intensity is low enough for non-linear effects caused by multiphoton absorption or level shifts such as the AC Stark effect to be insignificant. This was a given in the age of Einstein, well before lasers had been invented.)[citation needed]
Electrons can absorb energy from photons when irradiated, but they usually follow an “all or nothing” principle. All of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or the energy is re-emitted. If the photon energy is absorbed, some of the energy liberates the electron from the atom, and the rest contributes to the electron’s kinetic energy as a free particle.[citation needed]
[edit]Experimental results of the photoelectric emission
For a given metal and frequency of incident radiation, the rate at which photoelectrons are ejected is directly proportional to the intensity of the incident light.
For a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons can be emitted. This frequency is called the threshold frequency.
For a given metal of particular work function, increase in frequency of incident beam increases the intensity of the photoelectric current.
Above the threshold frequency, the maximum kinetic energy of the emitted photoelectron depends on the frequency of the incident light, but is independent of the intensity of the incident light so long as the latter is not too high [9]
The time lag between the incidence of radiation and the emission of a photoelectron is very small, less than 10−9 second.
The direction distribution of emitted electrons peaks in the direction of polarization (the direction of the electric field) of the incident light, if it is linearly polarized.[citation needed]
[edit]Mathematical description
The maximum kinetic energy Kmax of an ejected electron is given by</p>
<p>where h is the Planck constant, f is the frequency of the incident photon, and φ = hf0 is the work function (sometimes denoted W), which is the minimum energy required to remove a delocalised electron from the surface of any given metal. The work function, in turn, can be written as</p>
<p>where f0 is called the threshold frequency for the metal. The maximum kinetic energy of an ejected electron is thus</p>
<p>Because the kinetic energy of the electron must be positive, it follows that the frequency f of the incident photon must be greater than f0 in order for the photoelectric effect to occur.[10]
[edit]Three-step model
In the X-ray regime, the photoelectric effect in crystalline material is often decomposed into three steps:[11]
Inner photoelectric effect (see photodiode below). The hole left behind can give rise to auger effect, which is visible even when the electron does not leave the material. In molecular solids phonons are excited in this step and may be visible as lines in the final electron energy. The inner photoeffect has to be dipole allowed. The transition rules for atoms translate via the tight-binding model onto the crystal. They are similar in geometry to plasma oscillations in that they have to be transversal.
Ballistic transport of half of the electrons to the surface. Some electrons are scattered.
Electrons escape from the material at the surface.
In the three-step model, an electron can take multiple paths through these three steps. All paths can interfere in the sense of the path integral formulation. For surface states and molecules the three-step model does still make some sense as even most atoms have multiple electrons which can scatter the one electron leaving.[citation needed]
[edit]History</p>
<p>[edit]Early observations
In 1839, Alexandre Edmond Becquerel observed the photoelectric effect via an electrode in a conductive solution exposed to light. In 1873, Willoughby Smith found that selenium is photoconductive.[citation needed]
[edit]Hertz’s spark gaps
In 1887, Heinrich Hertz observed the photoelectric effect and the production and reception of electromagnetic waves. He published these observations in the journal Annalen der Physik. His receiver consisted of a coil with a spark gap, where a spark would be seen upon detection of electromagnetic waves. He placed the apparatus in a darkened box to see the spark better. However, he noticed that the maximum spark length was reduced when in the box. A glass panel placed between the source of electromagnetic waves and the receiver absorbed ultraviolet radiation that assisted the electrons in jumping across the gap. When removed, the spark length would increase. He observed no decrease in spark length when he substituted quartz for glass, as quartz does not absorb UV radiation. Hertz concluded his months of investigation and reported the results obtained. He did not further pursue investigation of this effect, nor did he make any attempt at explaining how this phenomenon was brought about.[citation needed]
[edit]Stoletov: the first law of photoeffect
In the period from February 1888 and until 1891, a detailed analysis of photoeffect was performed by Aleksandr Stoletov with results published in 6 works; four of them in Comptes Rendus, one review in Physikalische Revue (translated from Russian), and the last work in Journal de Physique. First, in these works Stoletov invented a new experimental setup which was more suitable for a quantitative analysis of photoeffect. Using this setup, he discovered the direct proportionality between the intensity of light and the induced photo electric current (the first law of photoeffect or Stoletov’s law). One of his other findings resulted from measurements of the dependence of the intensity of the electric photo current on the gas pressure, where he found the existence of an optimal gas pressure Pm corresponding to a maximum photocurrent; this property was used for a creation of solar cells.[citation needed]
[edit]JJ Thomson: electrons
In 1899, J. J. Thomson investigated ultraviolet light in Crookes tubes. Influenced by the work of James Clerk Maxwell, Thomson deduced that cathode rays consisted of negatively charged particles, later called electrons, which he called “corpuscles”. In the research, Thomson enclosed a metal plate (a cathode) in a vacuum tube, and exposed it to high frequency radiation. It was thought that the oscillating electromagnetic fields caused the atoms’ field to resonate and, after reaching a certain amplitude, caused a subatomic “corpuscle” to be emitted, and current to be detected. The amount of this current varied with the intensity and colour of the radiation. Larger radiation intensity or frequency would produce more current.</p>
<p>Lol… Charlie you finally made me laugh!</p>
<p>whom was that liking part for???
now that ive wasted half of the day im going to waste the other half too…nebody ol @ around 00.00?</p>
<p>I wonder where you got that from.</p>
<p>See above.</p>
<p>Don’t worry ans, I like you too.</p>
<p>i guess ive been blessed</p>
<p>yay someone likes me…
i was so lonely all alone when this angel quasi came around and said she likes me…she made my day…
im so happy…()cries() theyre the tears of joy silly</p>
<p>Oh, totally.</p>
<p>Go away, now.</p>
<p>why… have i done something bad…did i hurt your feelings???
ooh i so love being nasty…sadly my little bro is often at the receiving end</p>
<p>I still can’t get over the blog!! :D</p>