Mirrors work by the 1) efficient and 2) predictable reflection of light at all visible wavelengths. 1) Surfaces which are efficient reflectors are termed "lustrous", and include most metals, water and some other liquids. When light strikes an object there are three possible outcomes: a) absorption at most wavelengths, as with coloured, opaque materials - this corresponds to the light waves/particles at those wavelegths, being stopped b) refraction, or transmission, as with transparent materials, which leads to changes in course for non-normal incident rays - this corresponds to slowing of the light or c) reflection, as with lustrous, or shiny materials - this corresponds to a reversal in the incident velocity of the light. In order for option c) to come about, the entire visible range of the incident light must be matched by the electromagnetic excitation response at the surface of the material. Because this is a diffuse range, the response in the reflector must be caused by excitation of _combinations_ of many simple particles simultaneously, thus producing an exact match for all the applicable wavelengths. It should be stressed, that this will only occur with a system involving many excitable particles simultaneously, as individual particles only react individually at clearly defined, fixed, specific frequencies. Such a system, is, however, present, at the surface of metals (or to a lesser extent water) because these materials have a "pool" or layer of many mobile electrons able to respond en masse to electromagnetic stimuli (which also gives rise to their relatively high electrical conductivity) Efficient reflection at lustrous surfaces is therefore possible. 2) Because reflection is a reversal in velocity of incident light waves, wave theory predicts that reflected light leaves the surface under consideration in a well-defined manner - specifically the angle of the incident ray to the normal at the local point of contact is equal to that of the reflected ray, and in the same plane. Therefore surfaces which are polished to become flat and smooth (i.e. planar, globally and locally) are predictable reflectors. In addition, the predictable images formed in such an artefact will be a clear and faithful mirror-image (or carbon-copy) of the object being inspected, because of this preservation of angles throughout. To summarise: any smooth, flat, lustrous surface will work as a mirror.