Photoelectric Effect

Photoelectric EffectPhotoelectric Effect(How Einstein really became (How Einstein really became

famous!)famous!)

A.K.Gupta, PGT Chemistry, KVS ZIET A.K.Gupta, PGT Chemistry, KVS ZIET BhubaneswaerBhubaneswaer

Photoelectric EffectPhotoelectric Effect

Metal FoilA.K.Gupta, PGT Chemistry, KVS ZIET

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PhotoelectricPhotoelectric EffectEffect

Metal FoilA.K.Gupta, PGT Chemistry, KVS ZIET

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PhotoelectricPhotoelectric EffectEffect As As blueblue light strikes the metal foil, light strikes the metal foil,

electrons are ejectedelectrons are ejected..

A.K.Gupta, PGT Chemistry, KVS ZIET Bhubaneswaer

Let red light fall on the Let red light fall on the surface of the metal surface of the metal

What do you observe?A.K.Gupta, PGT Chemistry, KVS ZIET

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PhotoelectricPhotoelectric EffectEffect When When redred light hits the metal foil, the light hits the metal foil, the

foil does not emit electrons.foil does not emit electrons. Blue light has more energy than Blue light has more energy than redred

light.light. How could we get more energy into How could we get more energy into

the red light?the red light? Try increasing the brightness Try increasing the brightness

(intensity).(intensity).A.K.Gupta, PGT Chemistry, KVS ZIET

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PhotoelectricPhotoelectric EffectEffect Well, that didn’t work!Well, that didn’t work! Maybe its still not bright enough.Maybe its still not bright enough.



What do you observe now?A.K.Gupta, PGT Chemistry, KVS ZIET

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PhotoelectricPhotoelectric EffectEffect► It is still not working.It is still not working.►Can you guess what happens with Can you guess what happens with

brighter (intense) brighter (intense) blueblue light? light?




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PhotoelectricPhotoelectric EffectEffect►More blue lightMore blue light cause emission of cause emission of

more electronsmore electrons, but , but that doesn’t work that doesn’t work with redwith red..


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.


Wave theory is unable to explain these Wave theory is unable to explain these phenomena, as the energy depends on phenomena, as the energy depends on the intensity (brightness)the intensity (brightness)According to wave theory bright red According to wave theory bright red light should work!light should work!

BUT IT DOESN’T!BUT IT DOESN’T!A.K.Gupta, PGT Chemistry, KVS ZIET

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Einstein's ExplanationEinstein's ExplanationEinstein said that light travels in tiny packets Einstein said that light travels in tiny packets called “called “quantaquanta” .” .The energyThe energy of each quanta is given by of each quanta is given by equation.equation.

E=hvE=hv00EnergyEnergy

Planck’s constantPlanck’s constant

frequencyfrequency


PhotoelectricPhotoelectric EffectEffect►Each metal has a minimum energy needed Each metal has a minimum energy needed

for an electron to be emitted.for an electron to be emitted.►This is known as the This is known as the work function, work function,

W(=hvW(=hv00).).►So, for an electron to be emitted, the So, for an electron to be emitted, the

energy of the photon,energy of the photon, hv hv, must be greater , must be greater than the work function, than the work function, W(=hvW(=hv00))..

►The excess energy is the The excess energy is the kinetic energy, Ekinetic energy, E of the emitted electron.of the emitted electron.


PhotoelectricPhotoelectric EffectEffectEINSTEIN’S PHOTOELECTRIC EQUATION:-EINSTEIN’S PHOTOELECTRIC EQUATION:-

E= hv-W=hv-hv0


ApplicationsApplications

► The Photoelectric effect has numerous applications, for example The Photoelectric effect has numerous applications, for example night vision devices take advantage of the effect. Photons entering night vision devices take advantage of the effect. Photons entering the device strike a plate which causes electrons to be emitted, these the device strike a plate which causes electrons to be emitted, these pass through a disk consisting of millions of channels, the current pass through a disk consisting of millions of channels, the current through these are amplified and directed towards a fluorescent through these are amplified and directed towards a fluorescent screen which glows when electrons hit it. Image converters, image screen which glows when electrons hit it. Image converters, image intensifiers, television camera tubes, and image storage tubes also intensifiers, television camera tubes, and image storage tubes also take advantage of the point-by-point emission of the take advantage of the point-by-point emission of the photocathode. In these devices an optical image incident on a photocathode. In these devices an optical image incident on a semitransparent photocathode is used to transform the light image semitransparent photocathode is used to transform the light image into an into an ““electron image.electron image.”” The electrons released by each element The electrons released by each element of the photoemitter are focused by an electron-optical device onto of the photoemitter are focused by an electron-optical device onto a fluorescent screen, reconverting it in the process again into an a fluorescent screen, reconverting it in the process again into an optical imageoptical image


Applications: Night Vision Applications: Night Vision DeviceDevice

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Photoelectric Smoke DetectorPhotoelectric Smoke Detector

Source: http://www.bassburglaralarms.com/images_products/d350rpl_addressable_duct_smoke_detector_b10685.jpgA.K.Gupta, PGT Chemistry, KVS ZIET

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Solar panels are nothing more than a series of metallic plates that face the Sun Solar 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 enough which 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.


ThanksThanksAmar, Francois G. Amar, Francois G. The Photoelectric EffectThe Photoelectric Effect. 25 Sep 2003. Section of Chemistry 121 for fall 03. . 25 Sep 2003. Section of Chemistry 121 for fall 03.

11 May 2006 <http://chemistry.umeche.maine.edu/~amar/fall2003/photoelectric.html>11 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 20006 Mainland High School: Online Physics Labs. 11 May 20006 <http://online.cctt.org/physicslab/content/PhyAPB/lessonnotes/dualnature/<http://online.cctt.org/physicslab/content/PhyAPB/lessonnotes/dualnature/photoelectric.asp>photoelectric.asp>

Helmenstine, Anne Marie. Helmenstine, Anne Marie. Photoelectric & Ionization Smoke DetectorPhotoelectric & Ionization Smoke Detector. 25 Feb 2006. . 25 Feb 2006. About.com. 11 May 2006 About.com. 11 May 2006 <http://chemistry.about.com/cs/howthingswork/a/aa071401a.htm><http://chemistry.about.com/cs/howthingswork/a/aa071401a.htm>

Einstein, Albert. “Concerning an Heuristic Point of View Toward the Emission and Einstein, 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, 2006: Thomson 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>.


Photoelectric Effect

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