Lecture 24 (Ch. 28.9-28.10)

# Lecture 24 (Ch. 28.9-28.10) - Lecture 24 Quantum Physics...

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Lecture 24 Quantum Physics III

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Final Exam Wed June 11 10:30am-12:30am ~3 long answer [60%] ~3 short answer [20%] ~8 MT [20%] 2/3 new material (since midterm) Pre-midterm material will all be similar to problems you have seen before.
Physics Circa 1900: Two Puzzles The Photoelectric Effect Electrons liberated from metal when light shines on the metal -- Prediction : Electrons should be ejected for any frequency of incident light (radio, visible, x- rays) as long as the intensity of light is large enough. -- Experiment : No electrons are emitted if the incident light frequency is smaller than a cutoff frequency f c . The cutoff frequency depends on the type of metal. No electrons are ejected below this cutoff frequency regardless of how intense the light is! IR light Red Light UV light Blue light No e’s produced Metal Slab

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Einstein’s Solution – The Photon o In 1905, Einstein explained the photo- electric effect by assuming That light consists of particles (photons). The energy of each photon depends on the light’s frequency f: o Each photon absorbed by the metal transfers all of its energy to a single electron ( φ = “work function” of metal) o Predicts K max independent of Intensity and that no electrons emerge if hf < φ As observed (h is Planck’s Constant
Einstein’s Photoelectric Effect

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Planck’s Constant h o Planck’s constant h is small, but not zero: h = 6.626 × 10 -34 J s = 4.14 x 10 -15 eV s = 0.0000000000000000000000000000000006626 J s
Photons o Energy of a photon depends on its frequency o Similarly, its momentum also depends on frequency

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Photons o Energy of a photon h = 6.626 × 10 -34 J s = 4.14 x 10 -15 eV s
Green light of 546nm hits a metal surface. The electrons emerge with a maximum energy of 0.5eV. What is the work function of the metal? h = 6.626 × 10 -34 J s = 4.14 x 10 -15 eV s hc = 1240 eV nm

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An FM radio station has a power output of 150kW and operates at a frequency of 99.7Mhz. How many photons per second does the station transmit?
An FM radio station has a power output of 150kW and operates at a frequency of 99.7Mhz. How many photons per second does the station transmit?

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What is Light? o Light travels like a wave It Refracts Shows Interference Exhibits the Doppler effect o Light “hits” like a particle! o This particle-wave “duality” is intimately linked to the theory of Quantum Mechanics. ? According to Quantum Mechanics everything has particle/wave duality. It only “shows up” on very small length scales, e.g., atoms, electrons, light, etc.
The Dual Nature of Light o Young’s double-slit experiment had shown a century before that light behaves like a wave (diffraction) Maxwell later described this behavior as part of electromagnetism o Einstein showed that the photoelectric effect could only be understood by treating light like a particle o This apparent paradox is an example of the particle/wave duality of quantum physics

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Matter – Particle and Wave? o
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Lecture 24 (Ch. 28.9-28.10) - Lecture 24 Quantum Physics...

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