The anchoring phenomenon for this unit is a one-way mirror, which is a material commonly used in exterior building windows, interrogation rooms, research and medical offices, and exterior house windows. One-way mirrors are made by placing a layer of one-way mirror film onto transparent glass or plastic. While a regular mirror includes a thick layer of reflective material layered onto glass (which makes the mirror opaque), a one-way mirror has a very thin layer of reflective material, like silver, aluminum, nickel, or tin. This thin layer is added to the front of the glass or plastic film. Applying a thin layer creates a “half-silvered” material. Some parts of the glass or plastic film are covered with the reflective coating. However, because the coating is so thin, this leaves some parts of the glass or plastic film still exposed. The result is a material that has some transparent surfaces and some reflective surfaces.
Due to its structure, a one-way mirror will reflect slightly more of the light that shines on it than it transmits. When the one-way mirror film is used in a situation where light is shining from both sides, the material looks a lot like a tinted film because light is reflecting and transmitting from both sides. When light shines from only one side, the material acts like a mirror from the light side and acts like a window from the dark side. This is the puzzling aspect of the phenomenon used to motivate students’ learning in the unit.
Students figure out that the one-way mirror works the way it does because light entering our eyes from the light side of the system is mostly coming from light reflected off the silver structures in the one-way mirror. Thus, a person standing on this side of the material sees their own reflection. In order to fully understand this side of the system, students will learn to trace the light from the light source, reflecting on the person on this side of the system toward the one-way mirror, and reflecting back to the person’s eyes. Alternatively, students trace the path of light transmitting through the one-way mirror. This light comes directly from the light source, but also from reflected light from objects on the light side of the system. The transmitted light enters the eyes of people on the dark side of the system. In order to fully explain why people see one thing and not the other, students need to account for how eyes detect light inputs and how the brain processes signals from the eyes. On the light side of the system, the light input from the reflected light is the strongest signal to the brain; thus, the brain tells the person that
they see their reflection and do not see through the one-way mirror. On the dark side of the system, the stronger signal of light is the transmitted light from the light side of the system; thus, the brain tells the people that they see through the one-way mirror.
