Lecture 23 (Ch 28.4-28.8)

Lecture 23 (Ch 28.4-28.8) - Lecture 23 Quantum Physics II...

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Lecture 23 Quantum Physics II
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Physics Circa 1900: Two Puzzles 1. Thermal Radiation The “Ultraviolet Catastrophe”: -- Prediction : The spectrum of electromagnetic radiation coming off of a hot object should increase as the wavelength of the radiation gets smaller. -- Experiment : The intensity of radiation actually peaks at a finite wavelength and then goes to zero at very small wavelengths of light.
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Physics Circa 1900: Two Puzzles 2. 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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The observed thermal Radiation Spectrum o Wien’s Law: Peak Wavelength λ max T = 2.898 × 10 -3 m K Hotter objects emit more of their light at shorter wavelengths
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Black-body Example o What color are you? Thermal radiation from a human body with temperature T = 37.0°C = 310K has maximum intensity at what wavelength?
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Black-body Example o What color are you? Thermal radiation from a human body with temperature T = 37.0°C has maximum intensity at what wavelength? Solution: From Wien’s Law, λ max T = 2.898 × 10 -3 m K λ max = (2.898 × 10 -3 m K)/T λ max = (2.898 × 10 -3 m K)/(37.0+273.15) = 9.35 μ m = 9350 nm Infrared light! (IR light is what we feel as “heat”)
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Why can you see other people if their black body spectrum emits in a wavelength that you can’t see?
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Planck’s Hypothesis o In 1900, Max Planck tried to understand why the spectrum didn’t diverge. o Planck found that he could only get the right answer if the energy of the oscillating atoms were forced to have discrete values (either E1 or E2, not half-way in between) -- the oscillating charges could only emit radiation of a particular frequency in chunks: o In other words, an oscillator with frequency f can only have energies which are an integral multiple of h × f.
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