Unique stamp issued on 2 March 2015 by Liechtenstein Post on 2015 - International Year of the Light. This special stamp “Play of Light” permits the observer to conduct three experiments relating to the light phenomenon. The background of the stamp glows in the dark owing to its fluorescent colour. It turns into a source of light if it was exposed to sunlight beforehand.
The wave theory can be observed if the stamp is held up to a source of light. The micro-perforations in the paper become easily visible as the light waves are diffracted through the small openings. For the experiment on quantum theory, the light pink colour behind the ball was printed with a special colour. When irradiated with UV light, the colour turns from pink to violet.