3.3 Diffraction, Interference, and Polarization

Pre-Lecture Reading 3.3

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See Interference.
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Particles can interfere with each other, due to intermolecular forces, but they cannot interfere with themselves.
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Waves can interfere with themselves, constructively and destructively.
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The single-particle version of Young's double-slit experiment shows that light, as well as electrons, protons, neutrons, atoms, molecules, etc., although emitted and detected as particles, travel as waves.
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  In other words, particles travel as waves when we are not looking, but become particles again when we try to observe them!
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  This very nonintuitive—even disturbing—result tells us something fundamental about the nature of the universe, at least on very small scales.
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  Given this, this experiment is arguably the most important of the 20^th century, if not of all time.
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  This phenomenon is called wave-particle duality, and serves as the foundation of quantum mechanics.