When talking about iridescence, it helps to remember that light is a wave and can be described as a wavelength: the distance between identical points on the wave. Waves can also be described by their phase: the position of their crests (high points) and troughs (low points). When two waves have the same phase, their crests and troughs would align if placed on top of one another.
Whether you know it or not, you have likely been familiar with iridescence since you were a kid. You probably marveled at it while blowing bubbles with a wand or playing with bubbles in the bathtub. With a soap bubble, light first passes through the top layer of the bubble, where some of it is reflected, while some light continues through to the bottom layer, where again some of it is reflected. Depending on the time it takes the second reflection to join the first and several other factors, the two waves may or may not line up, or have the same phase. If the phase of the two waves is different by some multiple of one full wavelength, the two waves are said to have constructive interference. If the two waves differ by half a wavelength or an odd multiple of that, they have destructive interference.
Constructive interference is what happens in iridescence. It causes the two waves to complement each other and strengthen the reflection. The effects of iridescence create much more intense colors than ordinary pigments ever could. Destructive interference causes the two waves to cancel each other out and weaken the reflection, or destroy it altogether. Basically, when the crests and troughs of more than one wave of light match up, their powers are magnified. When they do not match up, they are mostly destroyed.
OK, you're saying, but butterfly wings look nothing like soap bubbles. So how do they do it? Continue reading to find out how iridescence plays out in butterfly wings.