The Heisenberg Uncertainty Principle
The photoelectric effect showed that waves behave like particles. A wave with a frequency f has a minimum packet, or quantum, of energy E = hf, where h is Planck’s constant. Compton showed that when hf is comparable to the rest mass energy mc2 of an electron, the scattering of electromagnetic radiation from electrons behaves like the scattering of one compact object from another. The particle-like behavior of light seems so prominent in these cases that the quantum of light has been given the particle-like name of “photon.” Individual photons can be detected with a photomultiplier tube; such detection also suggests a degree of localization in space that is characteristic of particles rather than waves.
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