Mirrors typically have three layers: a dark protective layer, a middle layer of metal (usually aluminum, silver or tin), and a glass layer to protect the metal. Glass is also useful because it is so smooth; the best reflections come from smooth surfaces. Reflections occur when light strikes a surface so smooth that it bounces back without being scattered or distorted. This is why metals are such good reflectors. Other substances, such as plastic, or rocks, absorb too much light to act as mirrors.
A mirror's reflection works in the same way. Light strikes an object in front of the mirror, then bounces to the mirror itself, then to your eyes. Since mirrors are smooth, shiny, and metallic, the light bounces back in the same direction it struck the mirror, so the image of the object is duplicated in your eyes.
The new cinema subtitling system is similar to a teleprompter. Teleprompters rely on a reflected image of the words that are visible in a mirror in front of the camera lens. The mirror is "half-silvered." This means it has a much thinner reflective coating than a standard mirror, so that only half of the light that strikes its surface will be reflected. The other half will shine straight through. The image from the video monitor is reflected by the mirror, and is reversed left to right so that the mirror image will appear correct to the user. Because it is semi-transparent, the mirror also allows much of the light from the scene being photographed to pass through its surface into the camera lens.
How those police one-way mirrors work?
A one-way mirror is also "half-silvered." The reason a suspect can't see the detectives in the next room is because the questioning room is kept brightly lit, so there is plenty of light reflecting back from the mirror's surface. The suspect sees only his own reflection. The other room is darker, so detectives can see the suspect.