 The process of alteration is dependent on the wavelength of light, with alteration only occurring at wavelengths longer than approximately 500 nm, in the visible light spectrum.In realgar's crystal structure, each arsenic atom is bonded to two sulphur atoms and one other arsenic atom.The As-As bonds are 30% weaker than the As-S bonds and certain wavelengths of lights interact with the crystal structure of realgar, breaking the weaker bonds between arsenic atoms.