Why is the speed of light constant?
The speed of light is not constant. It varies across different media. In a vacuum the speed of light is 299,792,458 m/s. In air with a refractive index of 1.003 the speed of light would be 299,700 km/s (roughly 90 km/s slower than it is in a vacuum).
Light’s speed in a vacuum is designated with the symbol c and is taken to be a universal constant. Its value is the same regardless of the frame of reference of the observer. One cannot add or subtract onto this value as one would do in ordinary low speed Galilean relativity problems.
Light is a type of Electromagnetic radiation. The Scottish Mathematical Physicist James Clerk Maxwell described the behaviour of electromagnetic waves in his famous Maxwell Equations that provided the framework linking our understanding of both the electric and magnetic field concepts.
in doing so Maxwell determined that this value c is in all actuality a function of two other constants - electric constant ε and the magnetic constant μ0. The two are linked by the relationship
c= 1/((ε μ0)^-0.5)
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