International Year of Light

As Christmas present I got this year some online ordered stamps including some from Liechtenstein.

This is one stamp I got:

International Year of Light (issued 02-03-2015)

Information about this special stamp given on the website of the Liechtenstein Post:
"The United Nations proclaimed 2015 as the International Year of Light: 1000 years ago it was proven for the first time that light sources emit light rays, exactly 200 years ago the wave character of light was discovered and the quantum theory of light is 110 years old.

Experimenting with light arouses scientific curiosity. The special stamp “Play of Light” (value: CHF 1.90) permits the observer to conduct three experiments relating to the anniversaries mentioned. 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."