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Physics 102: Lecture 22
Quantum Mechanics:
Blackbody Radiation,
Photoelectric Effect,
Wave-Particle Duality
Physics 102: Lecture 22, Slide 1
State of Late 19th Century Physics
• Two great theories
“Classical physics”
– 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
Lecture 28
– Einstein and relativity
– The quantum revolution Lectures 22-25
Physics 102: Lecture 22, Slide 2
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.
Physics 102: Lecture 22, Slide 3
Three Early Indications of Problems
with Classical Physics
• Blackbody radiation
• Photoelectric effect
• Wave-particle duality
Physics 102: Lecture 22, Slide 4
Blackbody Radiation
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)
Physics 102: Lecture 22, Slide 5
Blackbody Radiation Spectrum
Visible Light:
~0.4mm to 0.7mm
Higher temperature: peak intensity at shorter l
Wien’s Displacement Law:
lmaxT = 2.898x10-3 m·K
Physics 102: Lecture 22, Slide 6
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/l
Planck’s constant: h = 6.626 x 10-34 Joule sec
Physics 102: Lecture 22, Slide 7
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
Physics 102: Lecture 22, Slide 8
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.
Physics 102: Lecture 22, Slide 9
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.)
light
e–
metal
Physics 102: Lecture 22, Slide 10
Photoelectric Effect: Light Intensity
• What happens to the rate electrons are
emitted when increase the brightness?
Rate increases
• What happens to max kinetic energy when
increase brightness?
Nothing
light
e–
metal
Physics 102: Lecture 22, Slide 11
Photoelectric Effect: Light Frequency
• What happens to rate electrons are emitted
when increase the frequency of the light?
Nothing, but goes to 0 for f < fmin
• What happens to max kinetic energy when
increase the frequency of the light?
Increases
light
No e–
e–
metal
Physics 102: Lecture 22, Slide 12
e–
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=hf
h = 6.626 x 10-34 Joule sec
• Maximum kinetic energy of released electrons
K.E. = hf – W0
KE
hf
e–
Physics 102: Lecture 22, Slide 13
W0
ACT: Photon
A red and green laser are each rated at 2.5mW.
Which one produces more photons/second?
1) Red
Physics 102: Lecture 22, Slide 14
2) Green
3) Same
Quantum Physics and the WaveParticle 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
Physics 102: Lecture 22, Slide 15
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!
Physics 102: Lecture 22, Slide 16
Young’s Double Slit w/ electron
Jönsson – 1961
d
Source of
monoenergetic
electrons
Physics 102: Lecture 22, Slide 17
L
2 slitsseparated
by d
Screen a distance
L from slits
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?
Physics 102: Lecture 22, Slide 18
Young’s Double Slit w/ electron
One electron at a time
Merli – 1974
Tonomura – 1989
d
Source of
monoenergetic
electrons
L
Interference pattern =
probability
Same pattern for photons
Physics 102: Lecture 22, Slide 19
ACT: Electrons are Particles
• If we shine a bright light, we can ‘see’
which hole the electron goes through.
(1) Both Slits
Physics 102: Lecture 22, Slide 20
(2) Only 1 Slit
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!
Physics 102: Lecture 22, Slide 21
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
Physics 102: Lecture 22, Slide 22
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!
Physics 102: Lecture 22, Slide 23
Quantum Summary
• Particles act as waves and waves act as
particles
• Physics is NOT deterministic
• Observations affect the experiment
Physics 102: Lecture 22, Slide 24