I just noticed "mirror" was actually created by applying thin layer of
metal on top of glass.
The vast majority of modern mirrors are created by applying a thin layer of metal to the back side of the glass or transparent plastic.
With all the mirrors in my house and my car,
if I look closely at a speck of dust that recently fell through the air onto the front (outer) surface of the mirror,
I appear to see 2 specks of dust, separated by a small gap -- the actual speck "floating" on the transparent plastic or glass, and its reflection.
In fact, the polished metal is exactly how
mirror gets its properties - while glass itself is just for structure.
You might think that the metal is polished to a flat, reflective surface,
and then later the glass is attached to the metal.
With the vast majority of modern mirrors,
they are manufactured in the opposite order --
the glass is polished or otherwise manufactured to give an optically flat surface,
and then afterwards liquid metal is poured or painted or sputtered onto what will become the back side of the glass.
...
So why then this "structure" necessary? Was that because of the metal
layer being too thin to stand by itself?
Yes, that is one of the several reasons -- silver is expensive, so we try to use as little as possible to cover the entire mirror, and that thin foil of silver cannot stand up by itself.
But also, silver is much easier to scratch than glass, so using a harder material like glass helps prevent the mirror from getting scratched up.
And if that so, then it is possible to create detachable mirror,
right? Where metal layer can be detached from it's glass structure
I mean, post-consumer wise, commercial mirror today isn't that easy to
recycle.
I've heard that "front surface mirrors", also called "first surface mirrors", exist,
but they are rare and difficult to clean without damage.
They're used in a few telescopes and a few other precision instruments.
With a front-surface mirror, you would be right.
With a front-surface mirror,
it doesn't matter what material structure is behind it.
And you are right that if we simply made the structure behind it the same kind of metal
as the optical reflecting surface,
such a homogenous slab of material would be easier to recycle
than two very different materials -- metal and glass -- bonded together.
And its just because of this manufacturing configuration?
There are other reasons.
The purpose of a mirror is to faithfully reflect all the light that hits it.
For reasons I won't go into here, the higher the conductivity of the reflective material,
the more light it reflects (and the less light is absorbed into or transmitted through the material).
(One major exception is dielectric mirrors).
The most conductive material we know of (at room temperature), and hence the most reflective material we know of, is silver.
Alas, as you may have seen with real silver silverware, silver soon tarnishes when exposed to the air.
Sputtering (untarnished) silver onto glass or clear plastic lets us see the untarnished surface of the silver, and isolates that surface from the air to prevent it from tarnishing.
Compared to silver-backed glass,
all other techniques for manufacturing mirrors are generally reflect less light:
- Some front-surface mirrors use silver, which quickly tarnishes and becomes far less reflective (and generally optically irregular).
- Some mirrors use silver behind transparent plastic, which is more easily scratched and scuffed than glass.
- Some mirrors use some other non-silver metal, which inherently is less reflective than polished silver. (aluminum is used in some instruments that require a front-surface mirror, because they don't tarnish and are far more reflective than tarnished silver). (automobile mirrors often use chrome-backed glass, because chromium costs less and they don't need to be as reflective).
