Reversible @T06446@ of a molecular entity between two forms, A and B, having different absorption spectra, induced in one or both directions by absorption of electromagnetic radiation. The spectral change produced is typically, but not necessarily, of @VT07496@ colour and is accompanied by differences in other physical properties. The thermodynamically @S05900@ form A is transformed by @I03255@ into form B. The back reaction can occur thermally (photochromism of type T) or photochemically (photochromism of type P). The terms 'switch-on' and 'switch-of' are sometimes used to denote the two directions of the photochromic reaction.