for visible light is about 1.0003, so the speed of light in air is about {{#expr:299792.458*(1-1/1.0003) round 0 |km/s|mi/s|comma=gaps|sigfig=2}} slower than .

Notation

The speed of light in vacuum is usually denoted by a lowercase . The origin of the letter choice is unclear, with guesses including "c" for "constant" or the Latin celeritas (meaning 'swiftness, celerity'). The "c" was used for "celerity" meaning a velocity in books by Leonhard Euler and others, but this velocity

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