Hi all, I purchased this photochromic fluid I have been trying to get to the bottom of how it works. From wikipedia http://en.wikipedia.org/wiki/Photochromism For example, the spiro form of an oxazine is a colorless leuco dye; the conjugated system of the oxazine and another aromatic part of the molecule is separated by a sp³-hybridized "spiro" carbon. After irradiation with UV light, the bond between the spiro-carbon and the oxazine breaks, the ring opens, the spiro carbon achieves sp² hybridization and becomes planar, the aromatic group rotates, aligns its π-orbitals with the rest of the molecule, and a conjugated system forms with ability to absorb photons of visible light, and therefore appear colorful. When the UV source is removed, the molecules gradually relax to their ground state, the carbon-oxygen bond reforms, the spiro-carbon becomes sp³ hybridized again, and the molecule returns to its colorless state. So the when bond breaks and the ring opened does the red example above say then absorb all the frequencies except the photons of the red part of the spectrum we are seeing? Any help gratefully received.