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l15 - Announcements Reminder Schedule changes(see web page...

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ISP209s10 Lecture 15 -1- Announcements Reminder: Schedule changes (see web page) No class on Thursday 3/18 Exam 2 pushed back to Tues. 3/30 Today: Quantum Mechanics (Ch.13/14)
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ISP209s10 Lecture 15 -2- Sometimes light acts like a wave… Interference <=> waves Bright: Constructive interference Dark : Destructive interference
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ISP209s10 Lecture 15 -3- Sometimes light acts like a particle… E EM = 0, 1hf, 2hf, 3hf, 4hf, … h*f = energy of 1 photon EM fields are made of discrete bundles of energy (quanta or photons) h = 6.625E-34 Js Planck’s Constant Particle-like:
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ISP209s10 Lecture 15 -4- EM fields are “quantized” (photons) It’s tempting to think of photons as particles. But they show interference. Indeed, the EM wave is still a wave; it’s just that it can only lose energy in units of photons. 1 photon deposits Energy hf
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ISP209s10 Lecture 15 -5- Quantum Non-Locality Spread out EM field instantaneously “knows” it has lost 1hf of energy when the photon impacts the screen at a distant point. Einstein was horrified! How Can a distant point In space “know” Instantaneously?
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ISP209s10 Lecture 15 -6- Matter Waves • In 1923, de Broglie thought Nature should be symmetric. I.e., particles should display wave-like features (interference, etc.) Momentum p = mass*velocity = Sqrt(2*mass*Energy) de Broglie’s equation tells us the wavelength of the “particle wave”
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ISP209s10 Lecture 15 -7- Example: Baseball Wavelength What is the wave length of a 1kg baseball moving at 1 m/s? DATA: h = 6.625E-34 Js, This is a TINY number! No wonder baseballs don’t seem like waves to us. The “smallness” of h together with the “largeness” of everyday masses is why we don’t see quantum effects in day-to-day life.
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ISP209s10 Lecture 15 -8- Example: Electron Wavelength What is the wave length an electron with an energy of 30 keV? eV J keV eV keV kg Js E m h p h e 19 31 34 10 6 . 1 1000 30 10 11 . 9 2 10 625 . 6 2 ! ! ! " # # " # " = = = $ m 12 10 084 . 7 ! " = # DATA: m e = 9.11E-31 kg, h = 6.625E-34 Js, 1eV = 1.6E-19 J This number not so tiny. This means We can see electrons acting like Waves (e.g., the 2-slit experiment).
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ISP209s10 Lecture 15 -9- Matter Waves • At atomic scales, particles indeed interfere like waves Electron beam de Broglie was right!
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ISP209s10 Lecture 15 -10- Quantization of “Matter Fields” Light waves are waves in EM fields. They interfere. The EM fields are quantized and the “quanta” (discrete energy packets = hf) are called photons.
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