Ever heard of a one-way mirror? Here’s how it works 

Are you a crime thriller junkie who spends a lot of time watching detective shows? Well, then you are probably aware of a special kind of mirror present in the interrogation room when suspects are being questioned. This allows people to stand at the other side of the mirror and observe what’s going on in that room, while the suspect has no clue that he is being observed. Kind of fascinating, isn’t it? Well, that’s what happens in case of one-way mirror, also called semi-transparent and half-silvered mirror. Okay, but who even thought of it and how on earth does it work? Let us find these answers together. 

Half-silvering: The method used for a one-way mirror  
Well, not much is known about the inventor of a one-way mirror. However, it was in 1903 that the first patent for a one-way mirror was filed in the USA. But it was then known as a transparent mirror. Nonetheless, this reciprocal mirror has gained immense popularity over the years. 

Now, what exactly is a one-way mirror? Turns out, one-way mirror is a special kind of mirror that is reflective on one side and transparent on the other; or brightly lit on one end and dark on the other. Can you guess standing on which side allows you to see through the mirror? Well, of course the darkened side!  

Okay, but how does it work? To understand this, you first need to know how normal mirrors work. Turns out, they involve a process called silvering. In this method, a thin layer of silver (hence the name!) is applied on a piece of glass, by simply spraying a silver solution over it, until it is uniformly spread. Interestingly, in modern times, aluminium is also used in place of silver, as it is more cost-effective. Are you wondering why silver or a similar metal is used? Oh, for the simple reason that they are reflective in nature, meaning they reflect light so as to give an image of what’s in front of it.  

Now that you have understood how silvering works, let’s move on to half-silvering or what is required to make a one-way mirror. Here, an extremely thin layer of reflective silver coating is used that enables the glass to reflect only half the light that hits it, while the other half goes straight through. Pondering what it means? It simply means that the glass turns reflective on one side and transparent on the other, that too simultaneously.  

Here comes another question: How does this allow people on one side to see nothing but their own reflections, while people on the other side can see through the mirror? Well, that has got to do with lighting! 

What has light got to do with a one-way mirror?  
That’s actually very easy. To make one side act like a normal mirror, the area has to be properly lit. Why? Because the brighter the light, more reflective the mirror’s surface is. In contrast, the area on the other side of the mirror is left completely dark, to avoid any kind of reflection, as the goal for these people is to not see their own reflection, but to see through to the other side.  

In case you are thinking what happens if both rooms are well-lit, it’s fairly simply - the one-way mirror no longer remains a mirror, but turns into a window enabling people on both sides to see one another. How crazy is that!