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300,000 kilometers per second in a vacuum which has a refractive index of 1.0
it slows down to 225,000 kilometers per second in water refractive index of 1.3
and 200,000 kilometers per second in glass (refractive index of 1.5)
For Physics constant it is - 3 x 10 to the power 8
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Speed of light in diffrent units :-
~m / Meter (SI Unit of distance) -> 3*10^8 ms^-1 or mps or m/s (or) 2.99792458*10^8 ms^-1 or mps or m/s
~km / Kilometer (Unit of distance) -> 299,792 kms^-1 or kmps or km/s