Here is a simple thin film intereference system. You can adjust the thickness of the film layer, and choose among several scenerios of layers.
Intensity of Reflected Light
The graph on top shows the Intensity as a function of the angle (1 = maximum intensity .. thus IN PHASE or BRIGHT ... zero = no intensity .. thus OUT OF PHASE or DARK).
The "rainbow" on the bottom illustrates the wavelength colors of the top graph, and also indicates the BRIGHTS (white) and the DARKS (black) from the information in the boxes to the left.
This is the "given a thickness of a film in a particular layer system ... what visible wavelengths will constructively or destructively interfere upon reflection from the top and bottom surface of the film" type problem.