∇ ∇ × × ( ∇ ∇ × × E ) = ∇ ∇ × × ( − − ∂ ∂ B ∂ ∂ t ) = − − ∂ ∂ ∂ ∂ t ( ∇ ∇ × × B ) = − − μ μ 0 ε ε 0 ∂ ∂ 2 E ∂ ∂ t 2 ∇ ∇ × × ( ∇ ∇ × × B ) = ∇ ∇ × × ( μ μ 0 ε ε 0 ∂ ∂ E ∂ ∂ t ) = μ μ 0 ε ε 0 ∂ ∂ ∂ ∂ t ( ∇ ∇ × × E ) = − − μ μ 0 ε ε 0 ∂ ∂ 2 B ∂ ∂ t 2