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Physics 30 – Electromagnetic Radiation – Part 2 Wave-Particle Duality To accompany Pearson Physics PowerPoint Presentation by R. Schultz [email protected] 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 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 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 classical physics prediction • Also the relationship between f and intensity was weird – no red hot, yellow hot, white hot; primarily ultra high frequency radiation, uv and beyond! 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): E nh f 1 quantum 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 E h f 6.63 10 34 J s 4.74 1014 Hz 3.14 10 19 J question 5 E hf h c 1.50 eV 1.60 10 19 J eV 6.63 10 34 J s 3.00 10 8 m s 8.29 10 7 m 829 nm 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 • Demo with electroscope and (-) charge • Introduction to the photoelectric effect incoming “light” e- A - very low + voltage …… 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 14.2 The Photoelectric Effect • Measuring maximum kinetic energy of the photoelectrons: incoming “light” eV A + - voltage increased until current drops to 0 voltage direction reversed 14.2 The Photoelectric Effect • If Vstop is the voltage required to stop the photoelectric current, then Ek max qVstop • Observations: • Beyond fo, Ek max f Electrons have a range of Ek, those from near the surface of the metal have the most • “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 W h fo 14.2 The Photoelectric Effect • Einstein’s complete equation: Ek max hf W qVstop hf W • This is a great equation for graphical analysis If given a table of f and Ek max or f and Vstop Ek max Vstop W h m= ;b= q q x-int = fo m = h; b = -W x-int = fo f 14.2 The Photoelectric Effect • Millikan, in 1916, verified Einstein’s equation • Examples, SNAP, page 241 • Question 3 E k max hf W h c W 3.00 10 8 m s 4.14 10 eV s 1.70 eV 7 5.30 10 m 0.643 eV 1.60 10 19 J eV 1.03 10 19 J 15 or 14.2 The Photoelectric Effect W 1.70 eV 1.60 10 19 J eV 2.72 10 19 J Ek max h c W 3.00 108 m s 19 6.63 10 J s 2.72 10 J 7 5.30 10 m 1.03 10 19 J 34 • The first method is better when an answer in eV is required 14.2 The Photoelectric Effect • Question 6 λmax → minimum f = fo W h fo fo max 3.10 eV W 14 7.49 10 Hz 15 h 4.14 10 eV s 3.00 108 m s c 7 4.01 10 m 401 nm 14 fo 7.49 10 Hz 14.2 The Photoelectric Effect • Question 15 • Shortest wavelength radiation will produce maximum kinetic energy of electrons Ek max hf W h Ek max c W 8m 3.00 10 s 19 J 6.63 10 34 J s 2.30 eV 1.60 10 eV 4.0 10 7 m 1.3 10 19 J • 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 E mc 2 E m 2 c • In classical physics p mv substitute Einstein's mass equivalence into the expression E E E p 2 v , but for EMR v c, p 2 c c c c 14.3 The Compton Effect • For EMR: hf E h p or p or p c c • Change in λ for a scattered photon is given by h f i 1 cos mc 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 p h h 6.63 1034 J s 10 8.5 10 m 0.85 nm 25 kg m p 7.8 10 s • Question 7 Read this question carefully – it’s an electron, not a photon p mv 9.11 10 31kg 0.110 3.00 108 m s 3.01 10 23 kg m s 14.3 The Compton Effect • Example: Practice Problem 1, page 724 f i h 1 cos mc 6.63 10 34 J s f 1.0 10 m 1 cos 90 31 8 m 9.11 10 kg 3.00 10 s 11 f 1.2 10 11m 0.012 nm • 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: • De Broglie: p h for light h h p mv 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 6.63 10 34 J s h 12 4.0 10 m 27 5m mv 1.67 10 kg 1.0 10 s 14.4 Matter Waves and the Power of Symmetric Thinking • Evidence for the wave behaviour of electrons: Davisson and Germer G.P. Thomson Electron scattering producing interference patterns • 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 h x p 4 • 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 h • 4 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