You may well be familiar with the classic blue bottle experiment. Methylene blue is reduced to its colourless leuco form by glucose and then re-oxidised by oxygen when the bottle is shaken.
This version has methylene blue being reduced photochemically by light and iron II, which presumably is oxidised to iron III in the process. In the presence of bright light, the blue solution bleaches in a few seconds. When the light is turned off, the blue colour re-appears.