Spider orb webs are known to produce colour displays in nature, both in reflection and transmission of sunlight, under
certain illumination conditions. The cause of these colours has been the subject of speculation since the time of Newton.
It has also been the topic of observational interpretation and some experiment which has proposed diffraction by the fine
silks, scattering from rough/structured surfaces and thin film effects as the primary causes. We report systematic studies
carried out using the silks of Australian orb web weaving spiders. Studies of both white light and laser light
scattering/propagation by natural spider silks have definitively determined the primary cause of the colour displays is
rainbows that can be understood by the application of geometric optics combined with new knowledge of the optical
properties of the spider web strands, silks, and proteins as optical materials. Additionally, a range of microscopies
(optical, AFM, optical surface profiling) show the silks to be optically flat. Overall, spider silks emerge as fascinating
optical materials with high dispersion, high birefringence and the potential for future research to show they have high
nonlinear optical coefficients. Their importance as a bioinspiration in optics is only just beginning to be realised. Their
special optical properties have been achieved by ~136 million years of evolution driven by the need for the web to evade
detection by insect prey.