# The two-way mirror

Many years ago, I was lucky enough to work in “Launch Pad” in the science museum. There were lots of wonderful activities and the following blog posts are about the activities. Launch Pad has been replaced by the equally wonderful “Wonderlab”

https://www.sciencemuseum.org.uk/home

https://www.sciencemuseum.org.uk/see-and-do/wonderlab-equinor-gallery

The two-way mirror

In this activity you turned the handle to change where the lights shone. As the lights changed so did the image you saw on the glass. First you saw yourself, and then …… as if by magic you saw your friend, sitting on the other side of the glass. Just by changing the light you could make the mirror work in two ways.

How does it work?

The mirror worked in two ways because of the nature of glass and the way the light was used. Objects are only visible if it is lit up.

You can repeat this activity with any sheet of glass (like a window) because it reflects light as well as transmitting it. The exhibit glass was slightly better because it was partially mirrored so that its reflection was better than normal glass, but you should see the effect with normal glass.

When you change the lighting in the room with the glass you will see the two-way effect.

https://www.leydenscience.org/physics/electmag/2waymir.html

To understand how a two-way mirror works, imagine two people are using the glass, yellow and green.

When the lights are turned so, they only shine on yellow, green can see right through the glass onto the bright side. He sees yellow. What does yellow see?

Yellow sees himself because some of the light on his side is reflected off the mirrored surface before it reaches green.

If the light is on green instead of yellow then the opposite happens.

Something very interesting happens if the lights are half on green and half on yellow. On each side, some of the light will pass through the glass and some of it will be reflected. Both yellow and green will see a mixture of themselves and the other person.

You can try this for yourself with any piece of glass where you can control the lighting on either side. If you line up your eyes with a friend you can get a good mix.

Making faces can be fun with families. With your brother or sister, or your mum and dad you can see facial similarities as you slowly change the lighting on each side.

The two-way effect

You will probably have noticed the two-way effect in less spectacular fashion at home, on a dark night. If you have the lights in a room turned on and you look out of a window, you will see your own reflection. Some of the light does go through the window but a lot is reflected back into the room. If you then turn the lights off, you will see much more clearly outside as your reflection fades away.

You may also have seen, or worn mirror sunglasses on a very bright day. These work on the same principle. Because the light outside is bright, some comes through the thin mirror coating so that you can see. Very little light comes from behind the glasses, so someone looking at them just sees the mirrors.

Applications in technology

Television presenters use a device called an autocue which allows them to look straight into the camera and read their lines at the same time

https://en.wikipedia.org/wiki/Autocue

https://en.wikipedia.org/wiki/Teleprompter

Schematic representation: (1) Video camera; (2) Shroud; (3) Video monitor; (4) Clear glass or beam splitter; (5) Image from subject; (6) Image from video monitor

The presenter sees the text reflected in an inclined sheet of partially reflecting glass. The camera films the presenter through the glass without the text being visible. The shroud surrounding the camera and the back side of the glass prevents unwanted light from entering the camera.

The technology has continued to develop to the point that glass teleprompters and confidence monitors are used for speeches given at large gatherings.

https://en.wikipedia.org/wiki/Teleprompter#Presidential_(or_glass)_teleprompters

Joe Biden flanked by two glass teleprompters. The two teleprompter monitors are embedded in the floor of the podium.

https://avcitizen.com/2015/02/what-is-a-confidence-monitor/

https://www.sureguard.co.uk/window-film/solar-film/

You may have noticed that many tower blocks and offices have partially reflecting windows. They are used because radiated heat (infra-red radiation) can be reflected very much like light. With these windows installed people can see out but we cannot see in. Because infra-red radiation is reflected away from the building, the building will be kept relatively cool. In hot cities there are many buildings like this.

https://www.wisegeek.com/what-is-reflective-glass.htm

Theatres have used the two-way phenomena for many years. A favourite Victorian illusion was ‘Pepper’s Ghost’.

https://en.wikipedia.org/wiki/Pepper’s_ghost

Pepper’s ghost is an illusion technique used in the theatre, amusement parks, museums, television, and concerts. It is named after the English scientist John Henry Pepper (1821–1900) who popularized the effect in a demonstration in 1862. Examples of the illusion are the Girl-to-Gorilla trick found in old carnival sideshows and the appearance of “Ghosts” at the Haunted Mansion and the “Blue Fairy” in Pinocchio’s Daring Journey, both at the Disneyland park in California. Teleprompters are a modern implementation of Pepper’s ghost.

The essential device employed to stage the illusion is an optical beam splitter operated in reverse to combine two images towards the audience’s point of view. The beam splitter element is typically a large, flat sheet of ordinary glass. The image of one scene is reflected from the glass surface towards the viewers, and the image of a second scene is transmitted through the glass. The stage lighting is controlled to selectively illuminate the scenes, but not the glass itself, which is invisible to observers. The combined image is genuine and not an illusion; the illusion consists of the audience not detecting the glass.

Stage setup for Pepper’s Ghost. A brightly-lit figure out of the audience’s sight below the stage is reflected in a pane of glass placed between the performer and the audience. To the audience, it appears as if the ghost is on stage.

https://en.wikipedia.org/wiki/John_Henry_Pepper

John Henry “Professor” Pepper (17 June 1821 – 25 March 1900) was a British scientist and inventor who toured the English-speaking world with his scientific demonstrations. He entertained the public, royalty, and fellow scientists with a wide range of technological innovations.

Simple, quick scene changes are possible using a backdrop of net curtains. One scene can be played in front of the curtain with ordinary lighting. Then the lights can be turned on behind the curtains to revel another set quickly.

Other uses for two-way mirrors

Investigators of all sorts use two-way mirrors to observe people without knowing and/or for their own personal safety.

Psychologists have used two-ways mirrors for many years to investigate human and animal behaviour. The subjects of their experiments can behave more ‘naturally’ if they are not being watched. The psychologists sit in the next room (in the dark) and watch the behaviour (in the light) through a window. On the subjects side the glass is mirrored, they simply see a reflection.

The police use two-way mirrors in identity parades. The suspects are brightly lit while the viewer sits in a dark rom and looks through a two-way mirror. This keeps the viewer anonymous and safe.

John Henry “Professor” Pepper (17 June 1821 – 25 March 1900) was a British scientist and inventor who toured the English-speaking world with his scientific demonstrations. He entertained the public, royalty, and fellow scientists with a wide range of technological innovations.

Simple, quick scene changes are possible using a backdrop of net curtains. One scene can be played in front of the curtain with ordinary lighting. Then the lights can be turned on behind the curtains to revel another set quickly.

Other uses for two-way mirrors

Investigators of all sorts use two-way mirrors to observe people without knowing and/or for their own personal safety.

Psychologists have used two-ways mirrors for many years to investigate human and animal behaviour. The subjects of their experiments can behave more ‘naturally’ if they are not being watched. The psychologists sit in the next room (in the dark) and watch the behaviour (in the light) through a window. On the subjects side the glass is mirrored, they simply see a reflection.

The police use two-way mirrors in identity parades. The suspects are brightly lit while the viewer sits in a dark room and looks through a two-way mirror. This keeps the viewer anonymous and safe.

https://science.howstuffworks.com/question421.htm

https://youtu.be/4kKL32opewI

Mr Bean and a two-way mirror