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

2. Wave-Particle Duality
2 “clouds” in physics at the beginning of the 20th century:
Weird relationship between temperature of a material and the colour of light given off
Why the speed of light was unaffected by Earth’s motion through space

3. 14.1 The Birth of the Quantum
a glowing hot object will emit increasingly bluer light as it temperature increases
at “relatively” low temperatures it will be red hot, then yellow hot, and finally white hot
actual behaviour

4. 14.1 The Birth of the Quantum
classical physics could only predict that intensity would increase as frequency increased; it could not make a prediction about any relationship between temperature and frequency
Also the relationship between f and intensity was weird – no red hot, yellow hot, white hot; primarily ultra high frequency radiation, uv and beyond!
classical physics prediction

5. 14.1 The Birth of the Quantum
Planck, 1900, able to explain actual behaviour by saying that matter could radiate (and absorb) only certain amounts of energy (quanta): where f is the lowest frequency possible for that substance, h is Planck’s constant, and n is a whole number, 1, 2, 3 …….
1 quantum

6. 14.1 The Birth of the Quantum
Quantization explained true behaviour exactly, but even Planck didn’t accept it
Too radical – like saying a pendulum couldn’t swing starting at any level, only certain allowed ones
Quanta of light were later called photons

7. 14.1 The Birth of the Quantum
Examples:
SNAP, page 232
question 3
question 5
Do questions 4 and 6, page 232, SNAP

8. 14.1 The Birth of the Quantum
One key realization is that the higher the frequency or shorter the wavelength of a photon, the more energy it has

9. 14.2 The Photoelectric Effect
Demo with electroscope and (-) charge
Introduction to the photoelectric effect
incoming “light” e-
very low voltage …… + A -

10. 14.2 The Photoelectric Effect
Observations: when “light” of a certain minimum frequency (threshold frequency, fo) or higher was shone on cathode of tube, there was an immediate photoelectric current
Above fo, increasing intensity of light increases photoelectric current
Below fo, no current no matter how high intensity of “light” or how long the light is shone

11. 14.2 The Photoelectric Effect
Measuring maximum kinetic energy of the photoelectrons:
incoming “light” e-
voltage increased until current drops to 0 V + A -
voltage direction reversed

12. 14.2 The Photoelectric Effect
If Vstop is the voltage required to stop the photoelectric current, then
Observations:
Beyond fo,
“Light” intensity has no effect on Ek max
Electrons have a range of Ek, those from near the surface of the metal have the most

13. 14.2 The Photoelectric Effect
Einstein’s explanation (1905):
photons of light with energy E=hf, are spread along wavefronts of light approaching surface
release of an electron is result of a single collision of 1 photon with 1 electron
minimum photon energy for release of an electron is W, the work function, and

14. 14.2 The Photoelectric Effect
Einstein’s complete equation:
This is a great equation for graphical analysis If given a table of f and Ek max or f and Vstop f
Ek max
m = h; b = -W x-int = fo
Vstop
m = ; b =
x-int = fo

15. 14.2 The Photoelectric Effect
Millikan, in 1916, verified Einstein’s equation
Examples, SNAP, page 241
Question 3 or

16. 14.2 The Photoelectric Effect
The first method is better when an answer in eV is required

17. 14.2 The Photoelectric Effect
Question 6
λmax → minimum f = fo

18. 14.2 The Photoelectric Effect
Question 15
Shortest wavelength radiation will produce maximum kinetic energy of electrons
Do SNAP, page 241, questions 4, 7, 8, 10, 11, 16, 19

19. 14.3 The Photoelectric Effect
Photoelectric Effect Applet experiment

20. 14.3 The Compton Effect
Compton observed a change in momentum (a particle property) when X-rays scattered off electrons
According to Einstein
In classical physics

21. 14.3 The Compton Effect
For EMR:
Change in λ for a scattered photon is given by
where θ is the scattering angle and m is the mass of the electron it scatters off of

22. 14.3 The Compton Effect
Examples: SNAP, page 252
Question 3
Question 7 Read this question carefully – it’s an electron, not a photon

23. Example: Practice Problem 1, page 724
14.3 The Compton Effect
Example: Practice Problem 1, page 724
There are no SNAP problems using this formula, but it is on the Formula Sheet

24. 14.3 The Compton Effect
Do questions 1, 4, 6, 10, 11 from SNAP, page 252
Question 11 is easier than it looks!

25. 14.4 Matter Waves and the Power of Symmetric Thinking
De Broglie, 1924, if light can sometimes behave as a particle (photoelectric effect, black-body radiation, Compton effect) why couldn’t classical particles, like electrons, sometimes behave as waves??
Compton: for light
De Broglie: for particles

26. 14.4 Matter Waves and the Power of Symmetric Thinking
Read and discuss Then, Now, and Future, page 727
Examples: Practice Problem 1 (2nd set), page 728

27. 14.4 Matter Waves and the Power of Symmetric Thinking
Evidence for the wave behaviour of electrons: Davisson and Germer
G.P. Thomson
De Broglie’s concept of electron waves explains why electron energy in an atom is quantized
The particle in a box analogy on pages is interesting reading (you won’t be tested on this)
Electron scattering producing interference patterns

28. 14.4 Matter Waves and the Power of Symmetric Thinking
The Heisenberg Uncertainty Principle
Δ x = uncertainty in position
Δ p = uncertainty in momentum
You can never know with certainty where a particle is and what it’s doing at the same time
is very tiny, so this doesn’t affect us macroscopically

29. 14.4 Matter Waves and the Power of Symmetric Thinking
Check and Reflect, page 736, questions 1, 2, 5, 6
Discuss question 3

30. 14.5 Coming to Terms
Read pages

31. 14.4 Matter Waves and the Power of Symmetric Thinking