5 - MIT OpenCourseWare http:/ocw.mit.edu 5.112 Principles...

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MIT OpenCourseWare http://ocw.mit.edu 5.112 Principles of Chemical Science, Fall 2005 Please use the following citation format: Sylvia Ceyer and Christopher Cummins, 5.112 Principles of Chemical Science, Fall 2005 (Massachusetts Institute of Technology: MIT OpenCourseWare). http://ocw.mit.edu (accessed MM DD, YYYY). License: Creative Commons Attribution-Noncommercial-Share Alike. Note: Please use the actual date you accessed this material in your citation. For more information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms
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MIT OpenCourseWare http://ocw.mit.edu 5.112 Principles of Chemical Science, Fall 2005 Transcript – Lecture 5 All right. Last time what we had done is looked at the first evidence for the particle-like nature of radiation. And that evidence was a photoelectric effect. The evidence was that what you had to have was a photon or a particle of energy, a quantum of energy, a packet of energy in order to get an electron out. And that energy had to be at least the energy of the work function of the metal. And so for every packet you put in there you got one electron out. That is an example of the particle-like nature of radiation. But Einstein went on to show an even more convincing property of the particle likeness of radiation or a photon. And that is what he did was showed that a photon has moment. It has moment even though a photon does not have mass, although a photon does not have rest mass for those of you in the know in this area. And having momentum is very much a particle-like property, right? Because you know how to write down momentum. Momentum is mass times velocity. You've got a mass in here. That is a particle-like property. And, yes, I am starting out with the lectures notes from number four, which I didn't finish last time. That is a particle-like property. But what Einstein showed was, from the relativistic equations of motion what drops out from the relativistic equations of motion is the fact that a photon, at a frequency nu, has a momentum h nu over C. And because we know the relationship between nu and C, nu times lambda equals C, I can write the momentum of a photon as h over lambda. If you have some radiation, this is the photon momentum here. If you have some radiation, at a wavelength lambda, that radiation or those photons have this momentum P given by h over that wavelength.
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Now, that was a prediction from the relativistic equations of motion. And it took another eight, ten years or so before there actually was an experiment that demonstrated the momentum of a photon. And that experiment was called the Compton Experiment. What went on in that experiment is that an x-ray beam came into some material or some molecule, some atoms, and they could actually see the transfer in the momentum from the photon to the atom. Kind of like in this website from the University of Colorado here. This is just a cartoon of what is happening, but I got this photon done
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5 - MIT OpenCourseWare http:/ocw.mit.edu 5.112 Principles...

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