What is the speed of light?

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What is the speed of light?

The speed of light in vacuum is exactly 299,792,458 m/s (metres per second)

In 1983 the SI (Systeme International) defined a metre as:

  • The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

When people refer to the speed of light, they refer to the definition above - the speed of light in a vacuum.

The speed of light is normally rounded to 300 000 kilometers per second or 186 000 miles per second.

The speed of light depends on the material that the light moves through - for example: light moves slower in water, glass and through the atmosphere than in a vacuum. The ratio whereby light is slowed down is called the refractive index of that medium.

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In general, the difference in the speed of light in other mediums is ignored.

Earlier thoughts: How fast is the speed of light?

Hundreds of year ago people thought light travelled instantaneously. They thought so because after a military artillery fired at a large distance, they saw the flash immediately, but sound took a noticeable delay before you heard it.

Even in 1600 A.D. the famous Johann Kepler believed that the speed of last was instantaneous. According to him the vacuum of space did not slow the speed of light down.

Galileo was correct in saying ...

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