4-Wave Particle Duality of Radiation and Matter

# 4-Wave Particle Duality of Radiation and Matter - 5.111...

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5.111 Lecture Summary #4 September 16, 2009 Reading for today: Sections 1.5-1.6 Reading for Lecture 5: Section 1.3, Section 1.7 up to equation 9b Wave-Particle Duality of Radiation and Matter II. Light as a particle: the photoelectric effect The Photoelectric effect Experiment: What was known: Classical Mechanics predicts Observed Experimentally metal UV light ( ν ) e- K.E. = ½ mv 2 K.E. I K.E. ν K.E. ν 0 linear threshhold K.E. No e - s # e - s E 0 φ = work function of metal K.E. of e- for fixed ν for fixed Ι for fixed ν for fixed Ι

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5.111 Lecture Summary #4 September 16, 2009 Einstein analyzed plots of K.E. versus ν for several metals. He found that the data all fit the linear form: K.E. = ax + b With slopes a = 6.626 x 10 -34 Js intercepts b = -(6.626 x 10 -34 Js) ν 0 where the constant 6.626 x 10 -34 Js was the same constant that Planck had previously discovered, (Planck’s constant h) h = 6.626 x 10 -34 Js Einstein then proposed (postulated) (1905): (1) Light is made up of energy “packets”, called “photons” (2) The energy of a photon is proportional to the light frequency E h ν = where h = Planck’s constant = 6.626 x 10 -34 Js Einstein had “quantized” light, a “packet” of light was a “quantum” of light, and for the first time, he connected the energy of light with its frequency. metal 1 Rb K.E. ν 0 (3) 0 (2) 0 (1) -h ν 0 (1) -h ν 0 (2) -h ν 0 (3) metal 2 K metal 3 Na
5.111 Lecture Summary #4 September 16, 2009 This provided a new model of the photoelectric effect: K.E. = h ν φ = 0 = ( 0 ) where = 0 So the picture is now: A beam of particles of light impinges on the metal, and if the energy of the particle of light is large enough, the number of electrons emerging from the metal is proportional to the number of photons absorbed by the metal.

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4-Wave Particle Duality of Radiation and Matter - 5.111...

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