A mirror consists of a piece of glass with silver painted
on the back.
Why is it that light is reflected off the front of the mirror?
When light strikes the mirror how does it 'know' it is striking a
mirror and not a plain piece of glass?
Why doesn't the light pass into the glass and reflect off the back?
Also, is there any limit to how thick a piece of glass can be and
still work as a mirror?
The simple answer to your question is that only some of the light is
reflected of the front of the glass; most is reflected of the layer of
silver at the back. In other words, most of the light passes through the
glass, reflects off the silver, passes through the glass again, and then
re-enters the air.
You can prove this to yourself by putting a dot of paint on a mirror; you
will see the reflection of the dot, suspended above the reflecting surface.
The little bit of light that reflects off the glass is just the same as the
light reflecting off any pane of glass. It's easy to see your reflection in
a store window; the same thing will happen with the mirror glass. You
won't easily see this reflection, because it is washed out by the reflection
from the silver layer.
As for how thick the pane of glass can be to still function in a mirror,
well, it has to be thin enough to still be clear.
Richard E. Barrans Jr., Ph.D.
Assistant Director, PG Research Foundation
Darien, IL USA
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Update: June 2012