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In every cop drama there's a scene where a suspect is being questioned in an interrogation room. The room contains a large mirror, and behind that mirror the detectives and district attorneys are observing and arguing about the progress of the case. The mirror is a two-way mirror.

These kinds of mirrors aren't complicated. Light shines on them, and some fraction is reflected back while some fraction passes through. The suspect in the brightly lit room can't see the dark room beyond the mirror because the bright room light washes out the much smaller amount coming from the adjacent dark room.

We use the same concept in the lab. A device called a beam splitter is essentially a one-way mirror. They come in various types and styles for various purposes, but let's pretend we have one that reflects exactly half the incoming field and transmits the other half. I'll draw a very simplified schematic of this sort of mirror:

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