Tuesday, January 4, 2011

The Two-Way Mirror

I went to a friend’s house on new year’s day and she took me on a little tour of her house. Her balcony, curiously enough, had a stepper (exercise machine) and was enclosed with a two-way mirror. For the rest of the evening, I couldn’t sit still! I had to figure out how the bloody thing worked!
So, as soon as I got home, I logged on and checked it out online and walaa! The mystery of the working of the two way mirror was discovered! :D
There are two sides to a pane of glass. The front and the front-of-the-back.
In conventional mirrors, it is the front-of-the-back which is silvered. Therefore, when light falls on it, it is almost completely reflected, leaving anyone on the other side of the mirror with nothing to look at but the painted back end.
However, in a two-way mirror, it is the front that is covered in a very sparse ( not dense ) coating of acrylic or a reflective material. Further, the lighting of the two rooms that this two-way mirror forms an interface between is very important.
Let ‘A’ and ‘B’ be two people...’A’ the prisoner and ‘B’ the cop.
A||||B represents the two people and the mirror in between them.
The room in which ‘A’ is present is very brightly lit while ‘B’’s room is dimly lit. Now, the coating on the mirror, is very thin and would cover say half the molecules on the front surface of the mirror, allowing quite a bit of light to pass through.
Since A’s room has so much light in it, say half the light passes through the two-way mirror into B’s room, allowing B to view A through what seems now like a tinted glass, which is what it exactly is! But B’s room is dimly lit, therefore, the light passing through to A is insufficient for A to see B. A ends up seeing on the light that is reflected off the coating, which is an image of itself. Hence, to A, the two-way mirror seems like a mirror!
If the lighting of the two rooms were to be interchanged, i.e. A’s room be made dim and B’s room be made bright, the situation would be reversed. That is why in cop stories on TV, the prisoner in his room, would try to cup his hand over his eyes and peer through the glass, thus obstructing the bright light of his room so that he may see through! And in the case of my friend’s balcony, she wouldn’t want other people in the surrounding flats to be able to see her on the stepper during  the morning hours when it is much brighter outside ( courtesy the sun :) ) while at night, there wouldn’t be much light in the sky anyway to let anyone look in.

A good way to figure out if you’re stuck at the wrong end of a two-way mirror is to put your finger up against the mirror. If the reflection of your finger is right up against your real finger without any gap, then your up against a two-way mirror. In a conventional mirror, you would notice a gap between your finger and its reflection, because the reflective coating is on the front-of-the-back of the mirror and not on the front ( unlike a two-way mirror ).


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