I liked the early iridescent tiles at Blackwell. It was hard to capture the colour shifts on camera. The principle is that light reflects from the top surface and also from a layer just underneath. The light that goes to the layer underneath will have travelled a tiny bit further and so will be slightly out of phase with the reflection from the top layer. Because light is made of many wavelengths, which are the colours that we see, at different angles the path difference for one colour may become a whole wavelength so there is constructive interference and a colour is seen.
