The formula used for thin film interference is EPD = 2t + Φ where t represents the thickness of the thin film Φ is the net phase inversion between rays 2 and 5 0 λ ½ λ EPD represents the extreme path difference constructive interference: mλ where m {0, 1, 2, 3…} destructive interference: ½(2m -1)λ where m {1, 2, 3...}