Iridescence is an optical phenomenon produced by the structure of the object observed. Depending on the angle from which it is seen, the object appears to be of different colours.
Principle and examples of iridescence
Objects that cause iridescence are composed of microstructures. This is particularly the case of soap bubbles, CDs or even the elytra of some beetles and the wings of some butterflies.
The complex stacking of these microstructures (the layers of scales themselves are composed of layers) sends light of different wavelengths in each direction. Thus, depending on the angle from which it is viewed, the wavelength - and therefore the colour - seen by the observer is different.
The wings of some diurnal butterflies are iridescent. © wildcat dunny, Flickr, cc by 2.0