Lect21 - Le cture21: Lase rs, Atom Mole s, S s, cule olids,...

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Lecture 21, p 1 Lecture 21: Lasers, Atoms, Molecules, Solids, etc. Review and Examples + e r + e ψ even
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Lecture 20, p 2 Lasers Photons are emitted when the electrons in atoms go from a higher state to a lower state Conversely, photons are absorbed when the electrons in atoms go from a lower state to a higher state photon Emission photon Absorption Fermions and bosons: Electrons, protons, and neutrons are fermions. Two identical fermions cannot occupy the same quantum state. (exclusion principle) Photons (and many atoms) are bosons. Unlike fermions, bosons actually “prefer” (to be explained soon) to be in the same quantum state. This is the physical principle on which lasers are based.
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Suppose we have an atom in an excited state. Eventually (at some random time) it will emit a photon and fall to the lower state. The emitted photon will go in a random direction. This is called “ spontaneous emission ”. Suppose, however, that before the spontaneous emission occurs, another photon of the same energy (emitted by another atom) comes by. Its presence will stimulate the atom to emit its photon. Two identical photons! Photon emitted by some other atom We now have two photons in the same quantum state: the same frequency, the same direction, and the same polarization. As they travel, they will stimulate even more emission.
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Lect21 - Le cture21: Lase rs, Atom Mole s, S s, cule olids,...

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