Kanchanapusakit W.Mahidol University2025-09-292025-09-292025-10-01American Journal of Physics Vol.93 No.10 (2025) , 775-78000029505https://repository.li.mahidol.ac.th/handle/123456789/112318Editor's Note: When you hold a CD or a DVD under a lamp, you may sometimes see a colored line caused by the interference of light rays dispersed by the disk's grooves. This paper investigates why, when the lamp is sufficiently close to the disk, the colored line is not of a single color but instead exhibits a gradation of colors. This article could be useful as a classroom demonstration in undergraduate optics courses or as a puzzle one could ask students to solve.Physics and AstronomyArticleSCOPUS10.1119/5.02113332-s2.0-10501668901619432909