Photoelectric Effect

Photoelectric EffectPhotoelectric Effect
(How Einstein really became famous!)(How Einstein really became famous!)
A.K.Gupta, PGT Chemistry, KVS ZIETA.K.Gupta, PGT Chemistry, KVS ZIET
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Photoelectric EffectPhotoelectric Effect
Metal Foil
A.K.Gupta, PGT Chemistry, KVS ZIET
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PhotoelectricPhotoelectric EffectEffect
Metal FoilA.K.Gupta, PGT Chemistry, KVS ZIET
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 AsAs blueblue light strikes the metal foil,light strikes the metal foil, electronselectrons
are ejectedare ejected..
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Let red light fall on the surfaceLet red light fall on the surface
of the metalof the metal
What do you observe?
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 WhenWhen redred light hits the metal foil, the foillight hits the metal foil, the foil
does not emit electrons.does not emit electrons.
 Blue light has more energy thanBlue light has more energy than redred light.light.
 How could we get more energy into the redHow could we get more energy into the red
light?light?
 Try increasing the brightness (intensity).Try increasing the brightness (intensity).
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What do you observe?
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 Well, that didn’t work!Well, that didn’t work!
 Maybe its still not bright enough.Maybe its still not bright enough.
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What do you observe now?
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►It is still not working.It is still not working.
►Can you guess what happens with brighterCan you guess what happens with brighter
(intense)(intense) blueblue light?light?
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►More blue lightMore blue light cause emission ofcause emission of moremore
electronselectrons, but, but that doesn’t work with redthat doesn’t work with red..
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Summary of the resultSummary of the result
 1.The electrons are ejected as soon as light strikes on
the metal, i.e.; there is no time lag between strike of light
and ejection of electron.
 2.The number of electrons emitted is proportional to the
intensity (brightness) of the light.
3.For each metal there is a characteristic minimum
frequency ( threshold frequency, v0) below which
photoelectric effect is not observed.
4.at a frequency v > v0 , the ejected electrons come out
with certain kinetic energy. The kinetic energy of these
electrons increase with increase of frequency of the light
used.
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 Wave theory is unable to explain theseWave theory is unable to explain these
phenomena, as the energy depends on thephenomena, as the energy depends on the
intensity (brightness)intensity (brightness)
According to wave theory bright red lightAccording to wave theory bright red light
should work!should work!
BUT IT DOESN’T!BUT IT DOESN’T!
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Einstein's ExplanationEinstein's Explanation
Einstein said that light travels in tinyEinstein said that light travels in tiny
packets called “packets called “quantaquanta” .” .
The energyThe energy of each quanta is given byof each quanta is given by
equation.equation.
E=hvE=hv00
EnergyEnergy
Planck’s constantPlanck’s constant
frequencyfrequency
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►Each metal has a minimum energy neededEach metal has a minimum energy needed
for an electron to be emitted.for an electron to be emitted.
►This is known as theThis is known as the work function,work function,
W(=hvW(=hv00 ).).
►So, for an electron to be emitted, the energySo, for an electron to be emitted, the energy
of the photon,of the photon, hvhv, must be greater than the, must be greater than the
work function,work function, W(=hvW(=hv00))..
►The excess energy is theThe excess energy is the kinetic energy, Ekinetic energy, E
of the emitted electron.of the emitted electron.A.K.Gupta, PGT Chemistry, KVS ZIET
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EINSTEIN’S PHOTOELECTRIC EQUATION:-EINSTEIN’S PHOTOELECTRIC EQUATION:-
E= hv-W
=hv-hv0
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ApplicationsApplications
► The Photoelectric effect has numerous applications, for exampleThe Photoelectric effect has numerous applications, for example
night vision devices take advantage of the effect. Photons enteringnight vision devices take advantage of the effect. Photons entering
the device strike a plate which causes electrons to be emitted, thesethe device strike a plate which causes electrons to be emitted, these
pass through a disk consisting of millions of channels, the currentpass through a disk consisting of millions of channels, the current
through these are amplified and directed towards a fluorescentthrough these are amplified and directed towards a fluorescent
screen which glows when electrons hit it. Image converters, imagescreen which glows when electrons hit it. Image converters, image
intensifiers, television camera tubes, and image storage tubes alsointensifiers, television camera tubes, and image storage tubes also
take advantage of the point-by-point emission of thetake advantage of the point-by-point emission of the
photocathode. In these devices an optical image incident on aphotocathode. In these devices an optical image incident on a
semitransparent photocathode is used to transform the light imagesemitransparent photocathode is used to transform the light image
into aninto an ““electron image.electron image.”” The electrons released by each elementThe electrons released by each element
of the photoemitter are focused by an electron-optical device ontoof the photoemitter are focused by an electron-optical device onto
a fluorescent screen, reconverting it in the process again into ana fluorescent screen, reconverting it in the process again into an
optical imageoptical image
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Applications: Night Vision DeviceApplications: Night Vision Device
http://www.lancs.ac.uk/ug/jacksom2/A.K.Gupta, PGT Chemistry, KVS ZIET
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Photoelectric Smoke DetectorPhotoelectric Smoke Detector
Source: http://www.bassburglaralarms.com/images_products/d350rpl_addressable_duct_smoke_detector_b10685.jpg
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Solar panels are nothing more than a series of metallic plates that face the SunSolar panels are nothing more than a series of metallic plates that face the Sun
and exploit the photoelectric effect. The light from the Sun will liberate electrons,and exploit the photoelectric effect. The light from the Sun will liberate electrons,
which can be used to heat your home, run your lights, or, in sufficient enoughwhich can be used to heat your home, run your lights, or, in sufficient enough
quantities, power everything in your home.quantities, power everything in your home.
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ThanksThanks
Amar, Francois G.Amar, Francois G. The Photoelectric EffectThe Photoelectric Effect. 25 Sep 2003. Section of Chemistry 121 for fall 03. 11. 25 Sep 2003. Section of Chemistry 121 for fall 03. 11
May 2006 <http://chemistry.umeche.maine.edu/~amar/fall2003/photoelectric.html>May 2006 <http://chemistry.umeche.maine.edu/~amar/fall2003/photoelectric.html>
Blawn, Jeramy R. and Colwell, Catharine H.Blawn, Jeramy R. and Colwell, Catharine H. Physics Lab: Photoelectric EffectPhysics Lab: Photoelectric Effect. 10 Jun 2003.. 10 Jun 2003.
Mainland High School: Online Physics Labs. 11 May 20006Mainland High School: Online Physics Labs. 11 May 20006
<http://online.cctt.org/physicslab/content/PhyAPB/lessonnotes/dualnature/photoelectric.asp><http://online.cctt.org/physicslab/content/PhyAPB/lessonnotes/dualnature/photoelectric.asp>
Helmenstine, Anne Marie.Helmenstine, Anne Marie. Photoelectric & Ionization Smoke DetectorPhotoelectric & Ionization Smoke Detector. 25 Feb 2006. About.com.. 25 Feb 2006. About.com.
11 May 2006 <http://chemistry.about.com/cs/howthingswork/a/aa071401a.htm>11 May 2006 <http://chemistry.about.com/cs/howthingswork/a/aa071401a.htm>
Einstein, Albert. “Concerning an Heuristic Point of View Toward the Emission andEinstein, Albert. “Concerning an Heuristic Point of View Toward the Emission and
Transformation of Light.”Transformation of Light.” American Journal Of PhysicsAmerican Journal Of Physics 5 May 1965: 137.5 May 1965: 137.
Nave, Rod.Nave, Rod. HyperPhysicsHyperPhysics. 19 Aug. 2000. Georgia State University. 06 May 2006. 19 Aug. 2000. Georgia State University. 06 May 2006
<http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html> .<http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html> .
Thornton T., Stephen, and Rex, Andrew.Thornton T., Stephen, and Rex, Andrew. Modern Physics for Scientists and EngineersModern Physics for Scientists and Engineers. Canada :. Canada :
Thomson Brooks/Core, 2006Thomson Brooks/Core, 2006
Photoelectric EffectPhotoelectric Effect. 24 Apr. 2006. Wikipedia Free Encyclopedia. 05 May 2006.. 24 Apr. 2006. Wikipedia Free Encyclopedia. 05 May 2006.
<http://en.wikipedia.org/wiki/Photoelectric_effect>.<http://en.wikipedia.org/wiki/Photoelectric_effect>.
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Photoelectric Effect

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