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.) The waves of rays ri and ra interfere and produce a maximum. The indices of refraction are 11 = 1.56, n2 = 2.00, n3 = 1.33, and the
wavelength 1 = 612 nm. Find the third least thickness. Answer in nm. Please help!