CH30!1!6 Quantum Physics HANIM MJ

8/18/2019 CH30!1!6 Quantum Physics HANIM MJ

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Copyright © 2010 Pearson Education, Inc.

Chapter 30

Quantum Physics


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Units of Chapter 30

•

Blackbody Radiation and Planck’sHypothesis of Quantized ner!y

• Photons and the Photoelectric ffect

• "he #ass and #omentum of aPhoton

• Photon $catterin! and the Compton

ffect

• "he de Bro!lie Hypothesis and

%a&e'Particle (uality


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)t the end of the lesson* the students

should be able to+

)ckno,led!e Planck’s hypothesis that

radiation emitted and absorbed by the ,all of

a black body ca&ity is -uantized.

)ppreciate instein’s contribution to

-uantum theory and its relation to black body

radiation.

(escribe Photoelectric ffect* apply e-uationin&ol&in! ener!y of a photon* ,ork function

and ma/imum kinetic ener!y of an eected

electron.


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$tate the rest mass of a photon and its

momentum.

/plain Compton ffect -ualitati&ely.

(escribe the de Bro!lie hypothesis.


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30'1 Blackbody Radiation and Planck’s

Hypothesis of Quantized ner!y

)n ideal blackbody absorbs all the li!ht that is

incident upon it.

#ultiple

reflections causeabsorption


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/perimentally* if ,emeasure the intensity of

the electroma!netic

radiation emitted by an

ideal blackbody* ,e find+

•

"he temperature is increased2

• "he peak in the radiation shifts

to,ard hi!her fre-uency.


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↑

")rea under !raph





"his illustrates a remarkable e/perimental

findin!+

"he distribution of ener!y in blackbody

radiation is independent of the material from

,hich the blackbody is constructed itdepends only on the temperature* T .

"he peak fre-uency is !i&en by+





"he peak ,a&elen!th increases linearly ,iththe temperature 4color5.

Classical physics calculations ,erecompletely unable to produce this

temperature dependence* leadin! to

somethin! called the 6ultra&iolet

catastrophe.7





Classical predictions ,ere that the intensity

increased rapidly ,ith fre-uency* hence the

ultra&iolet catastrophe.

• (i&er!e to at hi!her f .•

BB radiates ,ith of • 8U9 catastrophe.

• Planck assume ener!y

is -uantized• (eri&e a cur&e in

a!reement ,ith

e/perimental results.





Planck disco&ered that he could reproduce the

e/perimental cur&e by assumin! that the

radiation in a blackbody came in -uantized

ener!y packets* dependin! on the fre-uency+

"he constant h in this e-uation is kno,n as

Planck’s constant+



30': Photons and the Photoelectric ffect

Planck relates ener!y -uantization in BB is due

to -uantized &ibration of atom in the ,all.

He did not think the -uantized ener!y come

from the li!ht itself.

instein su!!ested that the -uantization of

li!ht ,as real2 that li!ht came in small packets

or bundle of ener!y* called photons ,ith

ener!y of+




instein’s photon model+

#ore intense beam of li!ht ,ill contain morephotons

the ener!y of each photon does not chan!e.

Beam of light or particles of photons, each carry energy hf.

If f is constant, increasing I , result:

photons tightly packed.

At a given time, >photons pass a point, >photons shines on surface, &

↑

to the surface.




"he photoelectric effect occurs ,hen a beam

of li!ht strikes a metal* and electrons areeected.

ach metal has a minimum amount of ener!y

re-uired to eect an electron* called the ,ork

function* W 0.

;f the electron is !i&en an ener!y E by the

beam of li!ht* its ma/imum kinetic ener!y is+



1e9 < 1.=/10'1>?




"his dia!ram sho,s the basic layout of a

photoelectric effect e/periment.

Result+ electric current

#etal plate

ectin! electrons

@i!ht shines

)ttracted to a A&e

char!ed collector plate



30': Photons and the Photoelectric ffectClassical predictions+

1. )ny beam of li!ht of any color can eect

electrons if it is intense enou!h.

:. "he ma/imum kinetic ener!y of an eected

electron should increase as the intensity

increases.

bser&ations+

3. @i!ht must ha&e a certain minimum fre-uency*

cutoff fre-uency* f 0 in order to eect electrons.

. #ore intensity only results in more electrons of

the same ener!y 4E independent of intensity but

depend on frequency 5.




/planations+

1. ach photon’s ener!y is determined by itsfre-uency. ;f it is less than the ,ork function*

W o * electrons ,ill not be eected* no matter

ho, intense the beam.*minimum condition: E=W o




:. ) more intense beam means more photons*

and therefore more eected electrons.

• $odium and !old ha&e different cutoff fre-uencies 4due to different material5.• $lope of the t,o lines is the same2 h

30 2 Ph d h



30-2 Photons and the

Photoelectric Efect

30 2 Ph d h



30-2 Photons and the

Photoelectric Efect

30 3 "h # d # t f Ph t



30'3 "he #ass and #omentum of a Photon

"he total ener!y can be ,ritten+

$ince the left side of the e-uation must be

zero for a photon* it follo,s that the ri!ht side

must be zero as ,ell.

Photons al,ays tra&el at the speed of li!ht* &



30'3 "he #ass and #omentum of a Photon

Dinally* ,e can ,rite the momentum of a

photon in the follo,in! ,ay+

30 Ph t $ tt i d th C t


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30' Photon $catterin! and the Compton

ffect

"he Compton effect occurs ,hen a photon

scatters off an atomic electron.

• E'ray photon scatter from an

electron 4at rest5.

• Result+ chan!e of ,a&elen!th

for the scattered photon.

• 8Compton effect.

30 Ph t $ tt i d th C t


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30' Photon $catterin! and the Compton

ffect

;n order for ener!y to be conser&ed* the ener!yof the scattered photon plus the ener!y of the

electron must e-ual the ener!y of the incomin!

photon.

"his means the ,a&elen!th of the out!oin!

photon is lon!er than the ,a&elen!th of the

incomin! one+

30 F "h d B li H th i d %


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30'F "he de Bro!lie Hypothesis and %a&e'

Particle (uality

;n 1>:3* de Bro!lie proposed that* as ,a&es cane/hibit particle'like beha&ior * particles should

e/hibit ,a&e'like beha&ior as ,ell 4,a&e'particle

duality5.

He proposed that the same relationship bet,een,a&elen!th and momentum should apply to

massi&e particles as ,ell as photons+



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Particle (uality

;ndeed* ,e can e&en

perform Goun!’s t,o'

slit e/periment ,ith

particles of theappropriate

,a&elen!th and find

the same diffraction

pattern.



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Particle (uality

"hese ima!es sho, the !radual creation of anelectron diffraction pattern.

lectrons arri&e at

the screen at

random locations

;ncreasin! the

number of electrons

Resultin!

interference

pattern becomes

more e&idence.

$ f Ch t 30


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$ummary of Chapter 30

• )n ideal blackbody absorbs all li!ht incident

on it. "he distribution of ener!y ,ithin it as a

function of fre-uency depends only on its

temperature.

• Dre-uency of ma/imum radiation+

• Planck’s hypothesis+



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• @i!ht is composed of photons* each ,ith

ener!y+

• ;n terms of ,a&elen!th+

• Photoelectric effect+ photons eectelectrons from metal surface.

• #inimum ener!y+ ,ork function* W 0

• #inimum fre-uency+



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• Photons ha&e zero rest mass.

• Photon momentum* fre-uency* and,a&elen!th+

• Compton effect+ a photon scatters off an

atomic electron* and e/its ,ith a lon!er

,a&elen!th+



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• de Bro!lie hypothesis+ particles ha&e

,a&elen!ths* dependin! on their momentum+

• @i!ht and matter display both ,a&elike

and particle'like properties.

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CH30!1!6 Quantum Physics HANIM MJ