How does a two-way mirror work? There is a screen on one side of the mirror that reflects most of the light but allows some of it to pass through.
Two-way mirrors were invented in 1903. We often see them used in police movies, where the detectives are watching a criminal on the other side of the two-way mirror, waiting for them to slip up and do something incriminating. Two-way mirrors are also called one-way mirrors, although I don’t really understand why because every mirror is a one-way mirror. A two-way mirror goes in both directions whereas a one-way mirror only goes in one direction. When the invention was patented in 1903, it was called a transparent mirror, which makes a little more sense.
A two-way mirror (I’m going to keep calling it that) works on the same principle as a window at night. With a regular window, light hits the glass and passes straight past all of the molecules in the glass. Molecules can be excited if they are struck by energy and that makes their electrons jump to a higher orbit. When they release this energy and go back to their regular orbit, they release the energy as light or heat. Different molecules can be excited by different amounts of energy, but it has to be a specific amount of energy. The molecules in glass cannot be excited by the energy that visible light or UV light carry, so the light passes straight through the window without being stopped. However, not all of the light passes straight through. About 5% of the light waves hit a molecule in the glass and get reflected backwards. That is more than enough light going through the window for the window to appear transparent, but just enough light being reflected for the window to become a mirror at night. When you have the lights on in your room and it is dark outside, people from outside can easily see into your room, but you cannot see out of the window. This is because of that 5% that is being reflected. When it is daylight, 95% of the light from outside drowns out the 5% that is being reflected. At night, there is no light coming from outside so the 5% being reflected looks like a mirror and the 95% from inside the house sails through the glass as normal. This is basically how a two-way mirror works.
A regular mirror has a coating of silver on the back that reflects all of the light that hits it. A two-way mirror has a very thin coating of a metal like aluminium. This layer is so thin that it can let about 10% of the light that hits it through while reflecting the other 90%. If the lights are on in the secret room behind the two-way mirror, you would be able to see through in both directions because 90% of the light from both rooms would pass through the aluminium layer and it would be just like a regular window. However, if the lights are off in the secret room, 10% of the light from the bright room passes through the mirror into the secret room and 90 % of the light reflects back, creating the illusion of a mirror. There is no light coming from the secret room, so nothing passes back through the mirror.
If you think that there might be a two-way mirror in a room you are in, there are several ways of testing it. The first thing you can do is to see if the mirror is attached to the wall. This, of course, doesn’t mean that it is a two-way mirror, but, if it is, it will have to be attached to the wall because it is basically a window. The second thing you can do is switch off the lights in the room you are in. This might allow you to make out something on the other side of the glass. If that doesn’t work, press your face to the glass and cup your hands to cut out as much of the light from the room you are in as possible. If there is something on the other side of the glass, you might be able to make it out. If you have your smartphone with you, switch on the flashlight and press it to the glass. If it is a two-way mirror, your flashlight will illuminate some of that room. You can also tap on the mirror because a two-way mirror will sound hollow. Another way is to put your fingernail to the mirror. If it is a real mirror, the reflection is bouncing off the silver layer at the back of the mirror and the transparent glass will cause a space between your fingernail and the reflected fingernail. If it is a two-way mirror, the reflection is bouncing off the front of the glass and there won’t be a space. This isn’t always a guarantee though. And this is what I learned today.
Photo by RDNE Stock project: https://www.pexels.com/photo/woman-wiping-the-mirror-5591459/
Sources
https://www.reflectiv.com/en/blog/post/how-does-a-two-way-mirror-works.html
https://www.howitworksdaily.com/how-do-two-way-mirrors-work
https://en.wikipedia.org/wiki/One-way_mirror
https://science.howstuffworks.com/question404.htm