Hi, Right now I am solving some exercises in connection with wave optics. If we have three layers of different materials (ex. air, gasoline, water) and we send a light ray through these layers and we calculate the wave length of the reflected rays, then we use these formulas: D = N*l; (D = optical distance, l = wave length) If the ray reflects from the material with greater n, then we must D replace with D+l/2.... I am a little bit confused about my knowledge of these things. I don't have any problems with exercises, but I would like to understand the theory a little bit better Could anyone comment the meaning of N and relations between the rays reflected from the surface and bottom of the second layer? Is it possible that ray passes through the third layer?