There are actually two different kinds of mirrors, and the answer is different for each one.
-- Plain old everyday hand mirror, vanity mirror, bathroom mirror, makeup mirror, etc.
Opaque, reflecting silver coating is on the back of the glass. Light from your tongue or your teeth flows to the front surface of the glass, through the glass, out of the back surface of the glass, bounces off of the silver coating on the back, reverses its direction, enters the back surface of the glass, comes back through the glass again, leaves the front of the glass, goes into your eyes, and you can see your teeth or your tongue.
Both surfaces of the glass, as well as the glass in between the surfaces, are transparent. The silver coating on the back is opaque. I know that, because when I look at the back of a mirror, I can't see any light coming through it. The coating on the back is also reflective ... a big part of the reason why a mirror works.
-- Expensive mirrors used by astronomers and eye-doctors. Known as "first surface" mirrors.
Opaque, reflecting silver coating is on the <em>front</em> of the glass. Light
from your tongue or your teeth flows toward the front surface of the glass,
but never actually gets there. It bounces off of
the silver coating on the front of the glass, reverses its direction, goes into your eyes, and you can see your teeth or
your tongue.
The glass is transparent, but that doesn't matter, because the light never reaches the glass. It only goes as far as the opaque silver coating on the front, and is reflected from there.
