In the figure below, light is incident perpendicularly on a thin layer of material 2 that lies between (thicker) materials 1 and 3. (The rays are tilted only for clarity.) Part of the light ends up in material 3 as ray r3 (the light does not reflect inside material 2) and r4 (the light reflects twice inside material 2).
The waves of r and r4 interfere, and here we consider the type of interference to be either maximum (max) or minimum (min). For this situation, the
indexes of refraction are 11 = 1.68, n2 = 1.61, M3 = 1.50, the type of interference is max, and the thin-layer thickness L = 405 nm. Give the
wavelength, A, that is in the visible range. (Enter A as measured in air.) Answer in nm. Please help!