Lecture slides from a class introducing quantum mechanics to non-majors, giving an overview of black-body radiation, the photoelectric effect, and the Bohr model. Used as part of a course titled "A Brief history of Timekeeping," as a lead-in to talking about atomic clocks
1. Relativity
Einstein’s solution: Two principles
Principle of Relativity:
All of the laws of physics are the
same for any two observers
moving at constant relative speed
Principle of Constancy of Speed of Light:
All observers see the same speed of light, no matter their relative
velocities.
Requires re-thinking of basic physics from the ground up
Requires re-thinking of nature of time and space
Time moves at different rates for different observers
2. Quantum Mechanics
The other great theory of modern physics
Deals with very small objects
Electrons, atoms, molecules
Grew out of problems that seemed simple
Black-body radiation
Photoelectric Effect
Atomic Spectra
Produces some very strange results…
4. Blackbody Radiation
Max Planck, 1900
Developed mathematical formula for spectrum
Problem: Derivation of formula required a mathematical trick
Introduced idea of “quantum” of energy
Completely overturned classical physics
5. Blackbody Model
Imagine object as box with “oscillators” in walls
Small amount of light leaks out blackbody spectrum
What radiation exists in box?
“Standing wave” integer number of half-wavelengths
fit across the length of the box
Divide thermal energy of object among possible modes
Add up all allowed modes to get total spectrum
(Rayleigh-Jeans approach; slightly different than Planck, but simpler)
7. Ultraviolet Catastrophe
Problem: Lots and lots of ways to get short wavelengths
120
200 modes, 0.02L bins
Predicts huge
100
80 amount of light at very
short wavelengths
Number
60
40
20
0
0.0 0.2 0.4 0.6 0.8 1.0
Wavelength (box length)
8. Quantum Hypothesis
Planck’s trick:
Each mode has a minimum energy depending on frequency
Can only contain an integer multiple of fundamental energy
Modes with very short wavelength would need more than their
share of thermal energy
Amount of radiation drops off very sharply at short wavelength
11. Photoelectric Effect
Shine light on some object,
electrons come out
Discovered by Heinrich Hertz, 1887
Simple model: Shaking electrons
Predict: 1) Number of ejected electrons depends on intensity
2) Energy of ejected electrons depends on intensity
3) No obvious dependence on frequency
12. Photoelectric Effect: Experiment
Observations:
1) Number of electrons
depends on intensity
2) Energy of electrons DOES
NOT depend on intensity
3) Cut-off frequency:
minimum frequency to get
any emission
4) Above cut-off, energy increases linearly
with frequency
13. Photoelectric Effect: Einstein
Einstein, 1905: “Heuristic Model” of PE Effect
Particle model: “Light quanta” with energy
Some minimum energy to remove electron:
“Work Function”
Energy of emitted electron:
Take’s Planck’s “trick” seriously, runs with the idea
14. Photoelectric Effect: Einstein
Observations:
1) Number of electrons depends on intensity
Higher intensity More quanta
2) Energy of electrons DOES NOT depend
on intensity
Only one photon to eject
3) Cut-off frequency: minimum frequency
to get any emission
Einstein in 1921
Nobel Prize portrait
4) Above cut-off, energy increases linearly Cited for PE Effect
with frequency
15. Atomic Spectra
Atoms emit light at discrete, characteristic frequencies
Observed in 1860’s, unexplained until 1913
16. Bohr Model
1913: Neils Bohr comes up with “solar system” model
1) Electrons orbit nucleus in certain “allowed states”
2) Electrons radiate only when moving between allowed states
3) Frequency of emitted/absorbed light determined by Planck rule
Works great for hydrogen, but no reason for ad hoc assumptions
17. Matter Waves
Louis de Broglie: Particles are Waves
Electrons occupy standing wave orbits
Orbit allowed only if integral number of
electron wavelengths
h
Wavelength determined by momentum
p
Same rule as for light…
18. Matter Waves
de Broglie Waves:
h
p
Why don’t we see this?
Planck’s Constant is tiny
h = 6.626 10 –34 J-s
More significant for single atoms
145 g baseball, 40 m/s 87Rb, 200 m/s
= 1.1 10 –34 m = 0.02 nm
Insignificant for macroscopic objects Still small, but can
start to see effects
19. Electron Diffraction
Send electrons at two slits in a barrier:
Image and video from Hitachi:
http://www.hitachi.com/rd/research/em/doubleslit.html
22. Light as a Clock
Light: Electromagnetic wave
Extremely regular oscillation
No moving parts
Use atoms as a reference:
Performance: Lose 1s in 100,000,000 years