Mirrors are '
devices' that
reflect photons.
The first mirrors were slabs of reflective glass
of
vulcanic origin, later polished metal was used.
Modern mirrors are either metalized mirrors
or
dielectric mirrors.
Metalized mirrors consists of one or more
layers of
metal on a
glass substate. Normaly the glass is given
a thin layer of
nickel,
chromium or
titanium. On
this layer, a thicker layer of
aluminium or
silver is
applied. On normal mirrors the glass side is used to
reflect the light, so the backside gets a dark protective
coating to protect the metal from water. Mirrors
used in for example
scanners need a lower
distortion, so
the metal side is used as reflector, and gets a
transparent silicon layer instead. When light hits
the metal, the metal atom's electrons are
excited. This
creates an electric field that deflects the photons(since photons
are charged particles) of the light. But the
process absorbs
some of the light
energy, so this type of mirror is only about
95% effective.
Dielectric mirrors have a
nonconductive reflective material.
A pane of glass or a pool of water is a dielectric reflector,
though not very effective. To make a mirror several layers
of a plastic or glass dielectric are used. Each layer only reflects
a small amount of light. The thickness of the layers is
carefully chosen so the reflected lightwaves enforce each other.
These mirrors can have over 99% reflectivity, but are usually only
reflective at a limited
angle. But in the last five
years a dielectric
mirror type is developed that has a much
broader angle(especialy
in the IR range).
It consists of
alternating layers of
polystyrene and
tellurium.