About a few weeks back while I was in the dark room preparing small square pieces of x-ray films, I discovered something interesting. The x-ray films are used in plasma-focus diagnostics but that's outside of this story.
What happened was, after preparing those light-sensitive x-ray films, I have to keep them in a light tight pocket. Because x-rays can penetrate the pocket material (usually a black pvc or in my case, a home-made black paper envelope) the films contained inside the pocket will register an image depending on the intensity of the x-ray, just like ordinary films.
Now, making the paper envelope requires a type of adhesive, so I used masking tape. Since I did this in a dark environment, it led me to observe something I could not understand at first.
So this is what happened:
The moment force is applied to the peeled tape, a layer of tape is lifted from the roll. What I saw was a momentary emission of bluish-white light coming out from the contact between peeled tape and the roll. This light only emits when the tape is pulled in a sudden. Astounded by this discovery, I kept repeating it to see if there are changes when I applied force of different magnitude (strength). The colour of the light is the same, but the intensity increases with increasing applied force.
So that night I went online to search what was it and I came across this from nature.com. It appears, if I were to perform this in a vacuum condition I would get x-rays! (what's more is that the x-ray intensity would be high enough to take x-ray images of a human thumb!)
Now since I was working in a plasma technology lab, I asked my supervisor if she's able to provide me a chamber for this test (and they have x-ray detecting diodes too) but she said the set-up is too time costly and took this as a novelty phenomenon.
Well, If I have a lab myself.
It appears that visible light I saw was generated from an effect called triboluminescence, which is an effect where light is emitted when solid material is given a mechanical stress such that chemical bonds are broken/altered to produce light.
Detailed information can be obtained from Wiki.