(upbeat music)
- Hi, I'm Jonathan, this is Brain Stuff,
and today we're talking one way mirrors,
aka two way mirrors,
aka transparent mirrors.
You know, those things
you see in a crime drama
when one cops interrogating a suspect
while another watches through
a window that appears,
from the suspect's side, to be a mirror.
It's not magic, TV cops aren't wizards,
except sometimes they are,
but that's fiction, and
transparent mirrors are science,
specifically materials science and optics.
A regular old mirror,
the kind hanging over your bathroom sink,
is a sheet of glass holding
up an extremely thin layer
of reflective metal.
The metal comes in the
form of a metallic salt
which can be dissolved in liquid
and sprayed onto the glass
in a process called silvering.
That's because silver
nitrate was the first stuff
used for this process.
These days, most mirrors
are actually silvered
with aluminum which is
cheaper and sturdier.
But silvering doesn't
make perfect mirrors,
they reflect most light,
but a little is still transmitted through
infinitesimal gaps in the
reflective middle layer.
So, everyday mirrors
receive an opaque backing
like dark paint.
This stops cold any
photons that slip through
the metal layer and
protects it from scratches.
Without the backing, you'd be
able to faintly see the wall
behind the mirror.
But what if you purposely
made a mirror imperfect?
Manufacturers of transparent mirrors
spray an even thinner, less
dense layer of silvering
onto the glass, meaning
it reflects less light.
For example, let's say half the
light of an ordinary mirror.
The rest passes straight through the glass
like it's a window, which it is.
A transparent mirror
with its sparse silvering
and lack of backing is just
a very reflective window,
and it's a window from both sides.
So, how come the suspect
sees his reflection
but the cop sees the suspect?
It's a trick of the light.
The observer room is kept dark
while the observees room is
lit up like the Vegas strip.
So, on the cop's side,
more light is coming
through the glass than being
reflected from the room,
and from the suspect's side,
more light is reflecting from the room
than being transmitted through the glass.
It's similar to how when
late at night you have
all the lights on, you can't
really see out your window,
but someone outside could
see into your house.
And hey, people ask about this a lot.
If you ever wanna test a mirror
to see if it's transparent,
block the light around you
and try to peer through.
A bright flashlight can
help illuminate anything
that might be behind the mirror.
That's all we have time for today,
but I wanna know,
what's your favorite
magic or haunted mirror?
Tell me in the comments.
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Oh, and for lots more brainy stuff,
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