The speed of light is not constant in all circumstances. It varies depending on the medium through which light travels. In a vacuum, however, the speed of light is exactly 299,792,458 m/s. In air, which has a refractive index of approximately 1.0003, the speed of light is about 299,700 km/s—roughly 90 km/s slower than in a vacuum.
The speed of light in a vacuum is designated by the symbol c and is considered a universal constant. Its value is the same regardless of the frame of reference of the observer. Unlike ordinary low-speed problems involving Galilean relativity, one cannot simply add or subtract velocities from c.
Light is a form of electromagnetic radiation. The Scottish mathematical physicist James Clerk Maxwell described the behaviour of electromagnetic waves through his famous Maxwell equations, which provided a mathematical framework linking the concepts of electric and magnetic fields.
In doing so, Maxwell showed that the speed of electromagnetic waves in a vacuum is determined by two fundamental constants: the electric constant, ε0, and the magnetic constant, μ0. They are related to the speed of light by the equation:
c=1/(ε0μ0)0.5
Thus, the speed of light is not an arbitrary value. It emerges from the fundamental properties of electric and magnetic fields in a vacuum.
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