Physics 30 – Electromagnetic Radiation – Part 2 Wave-Particle Duality

Physics 30 – Electromagnetic Radiation – Part 2Wave-Particle Duality To accompany Pearson Physics PowerPoint Presentation by R. Schultz robert.schultz@ei.educ.ab.ca

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

actual behaviour 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

classical physics prediction 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!

1 quantum 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 …….

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

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

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

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

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

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

Electrons have a range of Ek, those from near the surface of the metal have the most 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

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

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

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

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

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

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

14.3 The Photoelectric Effect Photoelectric Effect Applet experiment

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

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

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

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

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

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

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

14.4 Matter Waves and the Power of Symmetric Thinking Electron scattering producing interference patterns 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 731-3 is interesting reading (you won’t be tested on this)

14.4 Matter Waves and the Power of Symmetric Thinking The Heisenberg Uncertainty Principle Δx = uncertainty in position Δp = 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

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

14.5 Coming to Terms Read pages 737 - 740

14.4 Matter Waves and the Power of Symmetric Thinking

Physics 30 – Electromagnetic Radiation – Part 2 Wave-Particle Duality