Physics 30 – Electromagnetic Radiation – Part 2 Wave-Particle Duality To accompany Pearson Physics PowerPoint Presentation by R. Schultz [email protected].

Physics 30 – Electromagnetic Radiation – Physics 30 – Electromagnetic Radiation – Part 2Part 2

Wave-Particle DualityWave-Particle Duality

To accompany To accompany Pearson PhysicsPearson Physics

PowerPoint Presentation by R. [email protected]

Wave-Particle DualityWave-Particle Duality

2 “clouds” in physics at the beginning 2 “clouds” in physics at the beginning of the 20of the 20thth century: century:

Weird relationship between Weird relationship between temperature of a material and the temperature of a material and the colour of light given offcolour of light given off

Why the speed of light was unaffected Why the speed of light was unaffected by Earth’s motion through spaceby Earth’s motion through space

14.1 The Birth of the Quantum14.1 The Birth of the Quantum

• a glowing hot object will a glowing hot object will emit emit increasingly bluer increasingly bluer light as it light as it temperature temperature increasesincreases

• at “relatively” low at “relatively” low temperatures it will be red hot,temperatures it will be red hot,

then yellow hot, and then yellow hot, and finally finally white hotwhite hot

actual behaviour


• classical physics could only predict that classical physics could only predict that intensity would increase as frequency intensity would increase as frequency increased; it could not make a prediction increased; it could not make a prediction about any relationship between about any relationship between temperature and frequencytemperature and frequency

• Also the relationship between f and Also the relationship between f and intensity was weird – no red hot, yellow intensity was weird – no red hot, yellow hot, white hot; primarily ultra high hot, white hot; primarily ultra high frequency radiation, uv and beyond! frequency radiation, uv and beyond!

classical physics prediction


• Planck, 1900, able to explain actual Planck, 1900, able to explain actual behaviour by saying that matter behaviour by saying that matter could radiate (and absorb) only could radiate (and absorb) only certain amounts of energy certain amounts of energy (quanta)(quanta)::

where where f f is the lowest frequency is the lowest frequency possible for that substance, possible for that substance, hh is is Planck’s constant, and Planck’s constant, and nn is a whole is a whole number, 1, 2, 3 …….number, 1, 2, 3 …….

E nhf 1 quantum


• Quantization explained true Quantization explained true behaviour exactly, but even Planck behaviour exactly, but even Planck didn’t accept itdidn’t accept it

• Too radical – like saying a pendulum Too radical – like saying a pendulum couldn’t swing starting at any level, couldn’t swing starting at any level, only certain allowed onesonly certain allowed ones

• Quanta of light were later called Quanta of light were later called photonsphotons


• Examples:Examples:

SNAPSNAP, page 232, page 232

question 3question 3

question 5question 5

Do questions 4 and 6, page 232, Do questions 4 and 6, page 232, SNAPSNAP

34 14 196.63 10 4.74 10 3.14 10E hf J s Hz J

8

19 34

7

3.00 101.50 1.60 10 6.63 10

8.29 10 829

msJ

eV

cE hf h

eV J s

m nm


• One key realization is that the higher the One key realization is that the higher the frequency or shorter the wavelength of a frequency or shorter the wavelength of a photon, the more energy it hasphoton, the more energy it has

14.2 The Photoelectric Effect14.2 The Photoelectric Effect

• Demo with electroscope and (-) chargeDemo with electroscope and (-) charge

• Introduction to the photoelectric effectIntroduction to the photoelectric effectincoming “light”

A+-

very low voltage ……

e-


• Observations:Observations: when “light” of a certain when “light” of a certain minimum frequency (threshold frequency, minimum frequency (threshold frequency, ffoo) or higher was shone on cathode of tube, ) or higher was shone on cathode of tube, there was an there was an immediateimmediate photoelectric photoelectric currentcurrent

• Above Above ffoo, increasing intensity of light , increasing intensity of light increases photoelectric currentincreases photoelectric current

• Below Below ffoo, no current no matter how high , no current no matter how high intensity of “light” or how long the light is intensity of “light” or how long the light is shoneshone


• Measuring maximum kinetic energy of the Measuring maximum kinetic energy of the photoelectrons:photoelectrons:

incoming “light”

A+ -

voltage increased until current drops to 0

V

e-

voltage direction reversed


• If If VVstopstop is the voltage required to stop the is the voltage required to stop the photoelectric current, thenphotoelectric current, then

• Observations:Observations:

• Beyond Beyond ffoo,,

• ““Light” intensity has no effect on Light” intensity has no effect on EEk k maxmax

maxk stopE qV

maxkE f

Electrons have a range of Ek, those from near the surface of the metal have the most


• Einstein’s explanation (1905):Einstein’s explanation (1905):

photons of light with energy photons of light with energy E=hf,E=hf, are are spread along wavefronts of light spread along wavefronts of light approaching surfaceapproaching surface

release of an electron is result of a single release of an electron is result of a single collision of 1 photon with 1 electroncollision of 1 photon with 1 electron

minimum photon energy for release of an minimum photon energy for release of an electron is electron is WW, the work function, and , the work function, and

oW hf


• Einstein’s complete equation:Einstein’s complete equation:

• This is a great equation for graphical This is a great equation for graphical analysisanalysisIf given a table of If given a table of ff and and EEkk max max or or ff and and VVstopstop

maxkE hf W stopqV hf W

f

Ek max

m = h; b = -Wx-int = fo

Vstop

m = ; b =

x-int = fo

Wq

hq


• Millikan, in 1916, verified Einstein’s Millikan, in 1916, verified Einstein’s equationequation

• Examples, Examples, SNAPSNAP, page 241, page 241

• Question 3Question 38

157

19 19

max

3.00 104.14 10 1.70

5.30 100.643 1.60 10 1.03 10

k

ms

JeV

chf W h W

eV s e

E

Vm

eV J

or


• The first method is better when an answer The first method is better when an answer in eV is requiredin eV is required

19 19

83

ma

4 197

19

x

1.70 1.60 10 2.72 10

3.00 106.63 10 2.72 10

5.30 101.03 10

k

JeV

ms

W eV J

ch W

J s Jm

J

E



λλmaxmax →→ minimum minimum ff = = ffoo

max

1415

87

14

3.107.49 10

4.14 103.00 10

4.01 10 4017.49 10

o

o

ms

o

W hf

eVWf Hz

h eV sc

m nmf Hz


• Question 15Question 15• Shortest wavelength radiation will produce Shortest wavelength radiation will produce

maximum kinetic energy of electronsmaximum kinetic energy of electrons

• Do Do SNAP, SNAP, page 241, questions 4, 7, 8, 10, page 241, questions 4, 7, 8, 10, 11, 16, 1911, 16, 19

8

max

ma3

x4 19

7

19

3.00 106.63 10 2.30 1.60 10

4.0 101.3 10

ms J

eV

k

k

chf W h W

J s eVm

E

J

E


• Photoelectric Effect Applet experimentPhotoelectric Effect Applet experiment

14.3 The Compton Effect14.3 The Compton Effect

• Compton observed a change in momentum Compton observed a change in momentum (a particle property) when X-rays scattered (a particle property) when X-rays scattered off electronsoff electrons

• According to EinsteinAccording to Einstein

• In classical physics In classical physics

22

EE mc m

c

2 2

substitute Einstein's mass equivalence into the expression

, but for EMR ,

p mv

E E Ep v v c p c

c c c


• For EMR:For EMR:

• Change in Change in λλ for a scattered photon is given for a scattered photon is given byby

where where θθ is the scattering angle and is the scattering angle and mm is is the mass of the electron it scatters off ofthe mass of the electron it scatters off of

or or hfE h

p p pc c

1 cosfi

hmc


• Examples: Examples: SNAPSNAP, page 252, page 252


• Question 7Question 7 Read this question carefully – Read this question carefully – it’s an electron, not a photonit’s an electron, not a photon

3410

25

6.63 10 8.5 10 0.85

7.8 10 kg ms

h h J sp m nm

p

31 8 239.11 10 0.110 3.00 10 3.01 10 kg mms sp mv kg


• Example: Example: Practice Problem 1Practice Problem 1, page , page 724724

• There are no There are no SNAPSNAP problems using problems using this formula, but it is on the Formula this formula, but it is on the Formula SheetSheet

34

1131 8

11

1 cos

6.63 101.0 10 1 cos90

9.11 10 3.00 10

1.2 10 0.012

i

ms

f

f

f

hmc

J sm

kg

m nm


• Do questions 1, 4, 6, 10, 11 from Do questions 1, 4, 6, 10, 11 from SNAPSNAP, , page 252page 252

• Question 11 is easier than it looks! Question 11 is easier than it looks!

14.4 Matter Waves and the Power of 14.4 Matter Waves and the Power of Symmetric ThinkingSymmetric Thinking

• De Broglie, 1924, if light can sometimes De Broglie, 1924, if light can sometimes behave as a particle (photoelectric effect, behave as a particle (photoelectric effect, black-body radiation, Compton effect) why black-body radiation, Compton effect) why couldn’t classical particles, like electrons, couldn’t classical particles, like electrons, sometimes behave as waves??sometimes behave as waves??

• Compton: for lightCompton: for light

• De Broglie: for particlesDe Broglie: for particles

hp

h hp mv


• Read and discuss Read and discuss Then, Now, and FutureThen, Now, and Future, , page 727page 727

• Examples: Examples: Practice Problem 1Practice Problem 1 (2 (2ndnd set), set), page 728page 728

34

1227 5

6.63 104.0 10

1.67 10 1.0 10 ms

J shm

mv kg


• Evidence for the wave behaviour of Evidence for the wave behaviour of electrons: Davisson and Germerelectrons: Davisson and Germer

G.P. Thomson G.P. Thomson

• De Broglie’s concept of electron waves De Broglie’s concept of electron waves explains why electron energy in an atom is explains why electron energy in an atom is quantizedquantized

• The particle in a box analogy on pages 731-The particle in a box analogy on pages 731-3 is interesting reading (you won’t be 3 is interesting reading (you won’t be tested on this)tested on this)

Electron scattering producing interference patterns


• The Heisenberg Uncertainty PrincipleThe Heisenberg Uncertainty Principle

• ΔΔ xx = uncertainty in position = uncertainty in position• ΔΔ pp = uncertainty in momentum = uncertainty in momentum

• You can never know with certainty where a You can never know with certainty where a particle is and what it’s doing at the same particle is and what it’s doing at the same timetime

• is very tiny, so this doesn’t affect us is very tiny, so this doesn’t affect us macroscopicallymacroscopically

4h

x p

4h


• Check and ReflectCheck and Reflect, page 736, questions 1, , page 736, questions 1, 2, 5, 62, 5, 6

• Discuss question 3Discuss question 3

14.5 Coming to Terms14.5 Coming to Terms

• Read pages 737 - 740Read pages 737 - 740


Physics 30 – Electromagnetic Radiation – Part 2 Wave-Particle Duality To accompany Pearson Physics PowerPoint Presentation by R. Schultz [email protected].

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