Chemistry / Chem 0091 · Atom · ~20 seconds
The Meter
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The meter, SI's unit of length, is the distance light travels in a vacuum during a fixed fraction of a second.
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Because the defining constant is light's speed, any lab with an accurate clock can reproduce it without a bar.
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Students often picture the meter as a physical bar in a vault — true until 1960, not today. (NIST/BIPM, Aug 20, 2026.)