the speed of light (1c)
100% of c (1c)Speed · definition
The quantity is exact as entered. Comparisons use the clearly stated specimen, model or nominal measurement.
Speed
Just how fast is the speed of light (1c)?
Toward light speed
the speed of light (1c)
DISTANCE IN ONE SECOND
the speed of light (1c)
The quantity is exact as entered.
100% of c (1c)Speed
0
299,800 km in 1 s
Light in vacuum
Light in vacuum is electromagnetic radiation that travels through empty space without any material medium.
100% of c (1c)Speed
0
299,800 km in 1 s
Each track shows distance covered in the same second. The small illustrations identify the movers; their sizes are schematic.
A little perspective
The speed of light (1c) is approximately equal to the speed of light in vacuum.
Measurements, assumptions & sources +
the speed of light (1c)
Speed: 100% of c (1c); definition.
The quantity is exact as entered. Comparisons use the clearly stated specimen, model or nominal measurement.
Light in vacuum
Speed: 100% of c (1c); exact.
Exactly 299,792,458 metres per second in vacuum. Light travels more slowly through materials.
NIST — 2022 CODATA fundamental constants
The marked speed is calibrated. Unmeasured illustration details are representative.
Measurement details
Why these numbers?
Compare the speed of light (1c) with the speed of light in vacuum. The notes below explain how each value was chosen.
Light in vacuum
100% of c (1c)Speed · exact
Exactly 299,792,458 metres per second in vacuum. Light travels more slowly through materials.
- Comparing
- Speed ↔ Speed
- Relative size
- The speed of light (1c) is approximately equal to the speed of light in vacuum.
How to read the visual
The diagram uses the recorded measurements. Details outside the marked spans do not set the scale.
Speed is distance per unit time; it does not specify the total distance travelled.
299,790,000 m/smetres per second
1.0793 × 10⁹ km/hkilometres per hour
670,620,000 mphmiles per hour
Mach 874,030Mach
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