Photons
Discrete concentrations of energy, apparently massless elementary particles, that move at the speed of light. They are the unit or quantum of electromagnetic radiation. Photons are emitted when electrons move from one energy state to another. (From Hawley's Condensed Chemical Dictionary, 11th ed)
Year introduced: 1994
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Subheadings:
Tree Number(s): G01.249.705, G01.358.500.505.650.782, G01.590.540.782, G01.750.250.650.782, G01.750.770.578.782
MeSH Unique ID: D017785