The previous chapter discussed the effects of interference in a simple “one-layer” system like an oil slick on water or in looking through a soap bubble. We see color because light of a certain wavelength travels through the layer and back at a precise path length that enables constructive interference (i.e. in-phase). Thus, at that one viewing angle we just see one color—all other colors (with different wavelengths) do not meet the geometric conditions for constructive interference (in-phase). If we look at a different angle (by moving our eyes, or moving the object) different colors will come in-phase. This phenomenon is called iridescence—the changing in the apparent color of an object as the viewing angle changes.