Using the transfer matrix method, determine the reflectivity spectrum, |E^(r)|²/|E⁽ⁱ⁾ |² vs k, for a periodic stack of 20 layers, alternating with d₁ = 500 nm, n₁ = 3, and d₂ = 750 nm, n₂ = 2, surrounded by air on both sides. Do you see a photonic band gap, that is, a range of frequencies with no propagating waves?