Determine eta and c
[eta,c] = etac(n)
vector of refractive indices = [n1,...,nM]
vector of characteristic impedances = [eta1,...,etaM]
vector of speeds of light = [c1,...,cM]
etac(n) calculates eta and c. mu = eta/c, epsilon = 1/eta/c. [eta0,c0] = etac(1) generates the vacuum values.
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