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

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
“Classical physics”
Lecture 28
Lectures 22-25

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. Three Early Indications of Problems with Classical Physics
Blackbody radiation
Photoelectric effect
Wave-particle duality

5. 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)
Note humans are ‘hot’ 300K so we emit light, just not much in the visible spectrum. Try infrared.

6. Blackbody Radiation Spectrum
Visible Light: ~0.4mm to 0.7mm
Note humans are ‘hot’ 300K so we emit light, just not much in the visible spectrum. Try infrared.
Classical theory at 3000 k: ultraviolet catastrophe (see p. 985 text)
Higher temperature: peak intensity at shorter l
Wien’s Displacement Law:
lmaxT = 2.898x10-3 m·K

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/l
Planck’s constant: h = x Joule sec
Note humans are ‘hot’ 300K so we emit light, just not much in the visible spectrum. Try infrared.

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. 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. 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. 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
greater intensity increases current
does not change maximum KE
Nothing
light e–
metal

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?
Nothing, but goes to 0 for f < fmin
higher frequency light increases max. KE
Below threshold freq, no current
Electrons emitted immediately, no delay as “energy is accumulated”
Increases
light
No e– e– e–
metal

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 = x Joule sec
Maximum kinetic energy of released electrons
K.E. = hf – W0
All puzzles explained with quantum theory. hf W0 KE e–

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. Quantum Physics and the Wave-Particle Duality I
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. 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. Young’s Double Slit w/ electron
2 slits-separated by d
Jönsson – 1961
Go to physics 2000 web site for JAVA version
Source of monoenergetic electrons L
Screen a distance L from slits

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. Young’s Double Slit w/ electron
One electron at a time d
Merli – 1974
Tonomura – 1989
Go to physics 2000 web site for JAVA version
Source of monoenergetic electrons L
Interference pattern = probability
Same pattern for photons

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. 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. 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. More Nobel Prizes! 1906 J.J. Thompson 1937 G.P. Thompson (JJ’s son)
Showing cathode rays are particles (electrons).
1937 G.P. Thompson (JJ’s son)
Showed electrons are really waves.
Both were right!

24. Quantum Summary Particles act as waves and waves act as particles
Physics is NOT deterministic
Observations affect the experiment