Topic 13. Mirrors, part 1.

Since metals make the best general-purpose reflectors, they are usually used to make mirrors. Silver has a very high reflectivity across the entire visible spectrum and is usually the first choice for a mirror; aluminum is almost as good. Both of these metals interact with the oxygen and with the other components in the atmosphere to form compounds (mainly oxides and sulfides) that are not metallic and do not reflect nearly as well as the pure metal itself. The metallic reflecting surface is therefore protected in some way. Silver mirrors usually have a pane of glass in front of them; aluminum mirrors are often coated with a thin layer of aluminum oxide. In both cases, the protective layer degrades the reflectivity somewhat but protects the underlying metallic surface from oxidation.

The simple model of a metal as a perfect reflector breaks down if the metallic layer is very thin (that is, only a few atoms thick), so that there is not much difference between the surface of the material, where the external electric field is concentrated and the interior, which we modeled as a field-free region. These mirrors are often called “half-silvered” because the thickness of the layer is adjusted so that about 50% of the incident light is reflected. Since very little light is absorbed, the other 50% must be transmitted, and these mirrors are often used as “one-way” mirrors.

If a one-way mirror is placed between two rooms that have very different levels of illumination, a person standing in the bright room cannot see through the mirror because the 50% of the light that is reflected on the bright side is greater than the 50% of the much lower level of illumination on the dim side. A person on the dim side, however, can easily see into the bright room because 50% of the light from the bright side is transmitted.

The same effect can be produced with ordinary window glass, even though it is not a 50/50 reflector. Window glass typically reflects about 4% of the light that strikes it, but even this small reflection may be significantly larger than the 96% of the light transmitted from the other side. Thus ordinary windows act as mirrors for people inside a house at night, while outsiders can easily see in. The reverse is true during the day, when the outside is much brighter than the inside. People inside the house can easily see out, whereas those outside cannot see in because they cannot detect the light transmitted from the interior in the presence of the 4% of the light outside the house that is reflected from the windows. The windows therefore look like mirrors to those outside.

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