is the speed of light (i.e. phase velocity) in a medium with permeability μ, and permittivity ε, and ∇ is the Laplace operator. In a vacuum, v ph = c 0 = 299 792 458 m/s, a fundamental physical constant. The electromagnetic wave equation derives from Maxwell's equations. In most older literature, B is called the magnetic flux density or magnetic induction. The following equations ∇ ∇ ⋅ ⋅ E = 0 ∇ ∇ ⋅ ⋅ B = 0 predicate that any electromagnetic wave must be a transverse wave, where the electric field E and the magnetic field B are both perpendicular to the direction of wave propagation.