The flash you see when opening certain envelopes (especially the
self-seal variety) is caused by triboluminescence.
Triboluminescence is the emission of light resulting from friction
generated by the vigorous crushing or tearing of certain substances.
Sometimes rapidly yanking a piece of Scotch tape off a roll can
produce a small amount of visible light, and the same mechanism causes
the well-known "sparking" effect of wintergreen candy when it is
crushed between the teeth. All these phenomena are much more
noticeable in a darkened room, of course.
Briefly, triboluminescence occurs when molecules are jammed together,
forcing some of the molecules' electrons out of their atomic fields.
The free electrons collide with nitrogen in the air, imparting energy
to the nitrogen molecules, which causes them to vibrate and emit
light. Most of the light emitted is in the ultraviolet range, but they
do emit a small amount of visible light as well, which is violet or
bluish in color since it is in the high end of the visible-light
The reason why some self-seal envelopes do this more than others is
that some adhesives have a greater tendency to absorb the ultraviolet
(short wavelength) light that is produced by the excited nitrogen;
they re-emit this as light in the visible spectrum, specifically
Here you will find an article that poses the same question you've
asked (about the flash of light seen when opening an envelope) and a
follow-up with some interesting references:
Here's an interesting discussion of triboluminescence as it relates to
the "sparking" of wintergreen candy:
The Straight Dope Archives
More candy-sparking, as investigated in a simple scientific
Homepage of Dr. Linda M. Sweeting, Towson University
Here's an explanation of triboluminescence, with several good links to
other reference materials:
My Google search strategy:
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