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Lect05 - Wave-Particle Duality Collector A electrons S1...

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Wave-Particle Duality V stop (v) 0 0.5 1 1.5 2 2.5 3 3.5 0 5 10 15 f 0 S 1 S 2 + V Collector Metal Surface electrons vacuum A f (x10 14 Hz)
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Overview Overview z Photoelectric Effect light as particles (“Quantization of EM waves”) z Photon momentum/Compton scattering z Wave-particle Duality z Weird Quantumness Midterm Exam next week will cover topics through Lecture 6, and will include Discussion, Homework and Lab topics through HW 3. Review Session 1-3 PM Sunday Feb. 4, in 112 Gregory Hall
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Wave Wave - - particle Duality for particle Duality for Light and Matter Light and Matter z In Physics 212 and the first 4 Lectures of Physics 214, we viewed “light” as a wave (c.f. Poisson spot) z Surprise: In the early 1900’s, it was discovered that light has “particle”-like properties in some situations! z Furthermore, “matter” (i.e., electrons, protons, etc.) was found to exhibit “wave-like” properties under certain circumstances z These two discoveries revolutionized science and technology. What was (some of) the evidence?
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Photoelectric Effect (1) z Electrons in a metal are “bound” by the energy Φ , the “work function”. If you shine light on a clean metal surface, electrons can emerge Æ the light gives the electrons enough energy (> Φ) to escape. z perform the experiment in vacuum z measure the flow of emitted electrons with an ammeter Binding potential z How will the current depend on I and f? We might expect: z Increasing intensity I should increase the current. (By increasing the electric field E, the force on electrons, F = eE, is increased, causing more electrons to be kicked out of the metal.) z Increasing frequency f shouldn’t matter much. Perhaps a decrease in current due to rapid oscillations. z With a weak light, there should be a time delay before current starts to flow (to build up enough energy) Electroscope
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Photoelectric Effect (2) z Experiment 1: Measure the maximum energy of ejected electrons z Bias the “collector” with a negative charge to repel ejected electrons z Increase negative bias voltage until flow of ejected electrons decreases to zero. ( Current = 0 at V = V stop ) z Measurement of V stop tells the max kinetic energy, KE max = eV stop . Incident Light (variable frequency f) + V Collector Metal Surface electrons vacuum A The “stopping voltage” is independent of light intensity! The Result: Therefore, increasing the intensity I does not increase KE ! Demo
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Photoelectric Effect (3) z Experiment 2: Measure the maximum energy vs. f Incident Light (variable frequency f) + V Collector Metal Surface electrons vacuum A V stop (v) f (x10 14 Hz) 0 1 2 3 0 5 10 15 f 0 The Results: z Stopping voltage V stop (and the maximum kinetic energy of electrons) decreases with decreasing f (linear dependence). z Below a certain frequency f o , no electrons are emitted, even for intense light! Makes no sense classically: Increasing E should have an effect.
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