A colored line on an optical disk

We analyze the wavelengths of a bright colored line on an optical disk illuminated by light rays at large angles with respect to the normal to the plane of the disk. The reflected rays constructively interfere, allowing only those with specific resonant wavelengths to be seen. We observe that, in certain circumstances, the color of the line noticeably varies along its length. To account for this phenomenon, we develop an equation for the wavelength that results in constructive interference at a given reflection point and validate it through a set of experiments.