For A to Cλ1hc=E0z2(nc21−nA21) And λ2hc=E0z2(nc21−nB21)So for A and Bλ3hc=E0z2(nB21−nA21)Clearly subtracting equation (ii) from equation (i)hc[λ11−λ21]=E0z2[nB21−nA21]=λ3hc⇒λ31=λ11−λ21⇒λ31=6000×20006000−2000=30001λ3=3000A˚