A compact disc under skimming light rays

The optical properties of a compact disc (CD) under “skimming” light rays have been analyzed. We have noticed that a clear green line can be detected when the disc is irradiated with light rays coming from a lamp in such a way that only those skimming the CD, held horizontally, are selected. We provide a physical interpretation of this phenomenon on the basis of elementary optics concepts. Extension of these concepts to digital versatile discs (DVDs) is given.