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Speed of light
Beginner
About 3×10⁸ m/s in vacuum.
c = 3×10⁸ m/sc = 299,792,458 m/s exactly (defines the metre). Light takes 8.3 minutes from the Sun, 1.3 s from the Moon. In media v = c/n.
Light travels 7.5 times around Earth in a second.
Nothing with mass reaches it.
In more detail
The speed of light in vacuum is c = 1/sqrt(μ0ε0) ≈ 3 × 10⁸ m/s. It follows from the properties of electric and magnetic fields predicted by Maxwell's equations.
Real-world uses
- Satellite communication delays depend on the speed of light.
- GPS positioning relies on the constant speed of light.
- Astronomers measure distances in light years, using the speed of light as the basic unit of travel.
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Frequently asked questions
Why is c constant?
It follows from the fixed values of ε0 and μ0 in vacuum.
Can light travel faster in some medium?
No. Light slows in matter, but c is the maximum in vacuum.
Why is the speed of light so important?
It is a fundamental constant, and it appears in relativity and in the speed of all electromagnetic waves.
Related topics in AC Circuits & EM Waves
Displacement current · EM wave generation · EM wave properties · EM wave equation · Relation between E and B · Energy of EM waves