Lecture 2 Notes 53750

Lecture 2 Notes 53750 - Lecture 2 Notes B A Rowland 53750...

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Lecture 2 Notes B. A. Rowland 53750 Blackbody Radiation The early 1900s were a good time for physicists. They thought they had it all figured out. Unfortunately, the problem of Blackbody Radiation was to prove difficult for the scientific paradigm of the time. Basically, the laws of physics (at the time) could not reproduce the experimental curves shown in the slide. They predicted that an Ultraviolet Catastrophe would occur, in that radiation with very short wavelengths would be produced in large amounts (basically, that a fireplace in a living room should be emitting lots of X-rays and gamma rays—but this doesn’t really happen)! This is because old physics assumed that energy could be lost in any amount (anywhere from the infinitesimally small to the grotesquely large). In order to get the smooth curves with a definite peak, Max Planck reasoned that radiation (like matter) could only be lost or gained through “chunks”—that is, there is a fundamental indivisible unit of light called the quantum (think photon from the Photoelectric Effect). Multiple quanta can be involved in the process, but never a fraction of a quantum (only a whole number). This leads to the quantum theory of matter. Making this change in the equations exactly reproduced the experimental result, and started a revolution in physics that is ongoing today! Blackbody radiation is interesting because of the concept of color temperature . Basically, if you know the temperature of an object, you can compute what the predominate wavelength of the EMR that should be emitted by that object (this is true regardless of the type of material—even humans have a λ p ). Looking at the curves, p is defined as the wavelength for which the intensity of radiation coming off the blackbody is at a maximum. You can see this if you think of heating a piece of metal—as the temperature increases, first it glows red, orange, yellow, blue and eventually white. Each color (which is p ) corresponds to a specific temperature, and we can compute both with the following equation: Important equation: p W T = , where W = 2.898 x 10 6 nm K, and T is the temperature (in Kelvins). Remember that to get from Celsius to Kelvin you employ the following equation: Kelvins = Celsius + 273.15. Practice Problem: The peak wavelength of the sun’s radiation is 483 nm. How hot is the surface of the sun? Wave-Particle Duality Here is a summary of experiments discuss thus far. Light: Young’s Diffraction Experiment (wave) and the Photoelectric Effect and Compton experiments (particle). Electrons/Protons: Thomson’s Plum Pudding and Rutherford’s Gold Foil (particle) and Davisson- Germer experiments (wave). It would appear that both light and electrons can display both wave and particle properties, given the
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appropriate condition. This led Louis de Broglie to theorize that perhaps all matter consists of particles and associated waves—the idea of a wave-particle duality! All matter is hypothesized to have an associated wave—this one of the foundations of quantum
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Lecture 2 Notes 53750 - Lecture 2 Notes B A Rowland 53750...

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