1. Physics 102: Lecture 22, Slide 1
Quantum Mechanics:
Blackbody Radiation,
Photoelectric Effect,
Wave-Particle Duality
Physics 102: Lecture 22

2. Physics 102: Lecture 22, Slide 2
State of Late 19th
Century Physics
• Two great theories
– Newton’s laws of mechanics, including gravity
– Maxwell’s theory of electricity & magnetism,
including propagation of electromagnetic
waves
• But…some unsettling experimental results
calls into question these theories
– Einstein and relativity
– The quantum revolution
Lecture 28
Lectures 22-25
“Classical physics”

3. Physics 102: Lecture 22, Slide 3
Quantum Mechanics!
• At very small sizes the world is VERY different!
– Energy is discrete, not continuous.
– Everything is probability; nothing is for certain.
– Particles often seem to be in two places at same time.
– Looking at something changes how it behaves.

4. Physics 102: Lecture 22, Slide 4
Three Early Indications of Problems
with Classical Physics
• Blackbody radiation
• Photoelectric effect
• Wave-particle duality

5. Physics 102: Lecture 22, Slide 5
Hot objects glow (toaster coils, light bulbs, the sun).
As the temperature increases the color shifts from
Red (700 nm) to Blue (400 nm)
The classical physics prediction was completely
wrong! (It said that an infinite amount of energy
should be radiated by an object at finite temperature)
Blackbody Radiation

6. Physics 102: Lecture 22, Slide 6
Blackbody Radiation Spectrum
Visible Light:
~0.4µm to 0.7µm
Higher temperature: peak intensity at shorter λ
Wien’s Displacement Law:
λmaxT = 2.898x10-3
m·K

7. Physics 102: Lecture 22, Slide 7
Blackbody Radiation:
First evidence for Q.M.
Max Planck found he could explain these curves if he assumed
that electromagnetic energy was radiated in discrete chunks,
rather than continuously.
The “quanta” of electromagnetic energy is called the photon.
Energy carried by a single photon is
E = hf = hc/λ
Planck’s constant: h = 6.626 x 10-34
Joule sec

8. Physics 102: Lecture 22, Slide 8
Preflights 22.1, 22.3
A series of light bulbs are colored red, yellow, and blue.
Which bulb emits photons with the most energy?
The least energy?
Which is hotter?
(1) stove burner glowing red
(2) stove burner glowing orange

9. Physics 102: Lecture 22, Slide 9
ACT: Nobel Trivia
For which work did Einstein receive the
Nobel Prize?
1) Special Relativity E=mc2
2) General Relativity Gravity bends Light
3) Photoelectric Effect Photons
4) Einstein didn’t receive a Nobel prize.

10. Physics 102: Lecture 22, Slide 10
Photoelectric Effect
• Light shining on a metal can “knock” electrons
out of atoms.
• Light must provide energy to overcome
Coulomb attraction of electron to nucleus
• Light Intensity gives power/area (i.e. Watts/m2
)
– Recall: Power = Energy/time (i.e. Joules/sec.)
metal
light e–

11. Physics 102: Lecture 22, Slide 11
Photoelectric Effect: Light Intensity
• What happens to the rate electrons are
emitted when increase the brightness?
• What happens to max kinetic energy when
increase brightness?
Rate increases
Nothing
metal
light
e–

12. Physics 102: Lecture 22, Slide 12
Photoelectric Effect: Light Frequency
• What happens to rate electrons are emitted
when increase the frequency of the light?
• What happens to max kinetic energy when
increase the frequency of the light?
Increases
Nothing, but goes to 0 for f < fmin
metal
e–
light
No e–
e–

13. Physics 102: Lecture 22, Slide 13
Photoelectric Effect Summary
• Each metal has “Work Function” (W0) which is the
minimum energy needed to free electron from atom.
• Light comes in packets called Photons
E = h f h = 6.626 x 10-34
Joule sec
• Maximum kinetic energy of released electrons
K.E. = hf – W0
hf
W0
KE
e–

14. Physics 102: Lecture 22, Slide 14
ACT: Photon
A red and green laser are each rated at 2.5mW.
Which one produces more photons/second?
1) Red 2) Green 3) Same

15. Physics 102: Lecture 22, Slide 15
Quantum Physics and the Wave-
Particle Duality
I. Is Light a Wave or a Particle?
• Wave
– Electric and Magnetic fields act like waves
– Superposition: Interference and Diffraction
• Particle
– Photons (blackbody radiation)
– Collision with electrons in photo-electric effect
BOTH Particle AND Wave

16. Physics 102: Lecture 22, Slide 16
II. Are Electrons Particles or Waves?
• Particles, definitely particles.
• You can “see them”.
• You can “bounce” things off them.
• You can put them on an electroscope.
• How would know if electron was a wave?
Look for interference!

17. Physics 102: Lecture 22, Slide 17
Young’s Double Slit w/ electron
Screen a
distance L from
Source of
monoenergetic
electrons
d
2 slits-
separated
by d
L
Jönsson – 1961

18. Physics 102: Lecture 22, Slide 18
Electrons are Waves?
• Electrons produce interference
pattern just like light waves.
– Need electrons to go through both slits.
– What if we send 1 electron at a time?
– Does a single electron go through both
slits?

19. Physics 102: Lecture 22, Slide 19
Young’s Double Slit w/ electron
Source of
monoenergetic
electrons
d
L
Merli – 1974
Tonomura – 1989
Same pattern for photons
One electron at a time
Interference pattern =
probability

20. Physics 102: Lecture 22, Slide 20
ACT: Electrons are Particles
• If we shine a bright light, we can ‘see’
which hole the electron goes through.
(1) Both Slits (2) Only 1 Slit

21. Physics 102: Lecture 22, Slide 21
Electrons are Particles and Waves!
• Depending on the experiment electron
can behave like
– wave (interference)
– particle (localized mass and charge)
• If we don’t look, electron goes through
both slits. If we do look it chooses 1.
I’m not kidding it’s true!

22. Physics 102: Lecture 22, Slide 22
Schrödinger's Cat
• Place cat in box with some poison. If we
don’t look at the cat it will be both dead and
alive!
Poison

23. Physics 102: Lecture 22, Slide 23
More Nobel Prizes!
• 1906 J.J. Thompson
– Showing cathode rays are particles (electrons).
• 1937 G.P. Thompson (JJ’s son)
– Showed electrons are really waves.
• Both were right!

24. Physics 102: Lecture 22, Slide 24
Quantum Summary
• Particles act as waves and waves act as
particles
• Physics is NOT deterministic
• Observations affect the experiment