The quantity μ μ 0 ε ε 0 has the dimension (T/L). Defining c = ( μ μ 0 ε ε 0 ) − − 1 / 2 , the equations above have the form of the standard wave equations 1 c 2 ∂ ∂ 2 E ∂ ∂ t 2 − − ∇ ∇ 2 E = 0 , 1 c 2 ∂ ∂ 2 B ∂ ∂ t 2 − − ∇ ∇ 2 B = 0.