The other portion enters the layer and travels on until it meets the lower surface, where again, some of it is reflected. These two reflected waves eventually combine to give the light that we observe. If the two waves reflect in phase with each other, constructive interference has occurred, and the light will be bright. If they are out of phase, they destructively interfere, and little or no reflected light will be seen. If, instead of a single cuticle layer, there is a whole series of parallel layers as in the complex Morpho structures, the constructive interference between reflected waves is stronger, and the consequent colours will be purer and even more intense.
Multilayer interference is only part of the story behind highly visible iridescence.
Dr Vukusic is also looking into the effects of diffraction , whereby the light waves scatter after passing through the periodic cuticle ridging on the scales, in much the same way that sea waves spread out after passing through a gap in a harbour wall. These periodic structures that cause the diffraction appear to be involved in broadening the angle of the reflected light so that the butterfly becomes highly visible from a wide variety of positions. The nature of the iridescent reflection depends not only on the relative thickness of the interspersed cuticle and air layers, but also on the angle from which you view the wing. Eventually, if you look at it almost horizontally, close to the grazing angle, the reflected light enters the ultraviolet spectrum, which is outside the range of human eyesight, and the wing appears dark brown. The scientists have also examined the optics of these systems by using lasers to measure exactly how much light a single scale can reflect, and to analyse the dispersal of that reflected light.