What is triboluminescence?
Triboluminescence is the name of the optical phenomenon when asymmetrical crystal bonds are broken in f.ex. the glue that keeps envelopes closed, the light emission comes from the separation and reunification of electric charges between the two surfaces that the glue is broken between. The light emission from envelopes are often blue.
The phenomenon can also be observed by rubbing a quartz crystal hard with another stone, the light from quartz crystal is often white /yellow.
The first scientific description of triboluminescence was done by Francis Bacon in 1620, he described how light was emitted from the breaking of sugar crystals.
In 1953 Russian scientists proved that triboluminescence in vacuum would result in x-ray emissions, in 2008 American scientists took it further by producing useful x-ray images from normal adhesive tape being peeled in a vacuum.
How can you perform this experiment yourself?
Save a stack of envelopes which you have cut open using a knife, so the glued lid is still on the envelope.
In a dark room the lid is pulled off in a controlled manner so the paper is not torn apart, a blue light can now be seen from the area where the glue is being broken between the paper.
Old envelopes and envelopes that uses glue that dries out are not suitable for this experiment.
This phenomenon is often mistaken for being a security feature of letters, so its possible to see if the letter have been opened prior to receiving it. But as the effect can be reproduced with the same glue a couple of times or by applying enough pressure to seal it properly, its about the same output for each try.
This is something that is easy to explain to your friends and almost everyone have experienced it when opening the mail in a dark hallway.