To this point we’ve discussed color originating from pigments and scattering, but color can also appear when light waves interact through interference. The interaction that we’re talking about refers to the “phase” of the light waves. When waves of light (or any kind of wave) are “in-phase,” the peaks and troughs of the two waves line up perfectly (Figure 16.1). The intensity of the two waves add together which means the intensity of the combined wave is increased. This is called constructive interference. For waves of light, that means the brightness of the light increases, or for sound waves that would be mean the sound would be louder. In contrast, if the peaks and troughs of two waves do not line up, the intensity of the combined wave can be reduced or if they are “out-of-phase” the intensity is perfectly cancelled out (Figure 16.2). The net result would be the absence of light (i.e. black) or for sound waves it would be the lack of sound (which is basically how noise cancelling headphones work). This is called destructive interference.