Chances are your bathroom mirror is a rear surface mirror -- where the aluminum or silver coating is on the back of the glass. With a rear surface mirror the light from the object being viewed passes through the glass to the back of the glass and then is reflected out through the glass again to viewer.
There are a couple of optic problems with this design. First, some of the light is reflected off the surface of the glass before it ever gets to the reflective coating on the back. Second, the light must pass through the glass twice (in and out) as it is reflected. For most mirrors (like that one in your bathroom) the degradation of the image is so slight it isn't a problem -- or even noticeable.
That isn't the case with some applications -- including the best kaleidoscopes -- where the image distortion can be a problem. The solution? First surface mirrors put the metallic surface on the front of the mirror so the light never passes through the glass. In fact, some first surface mirrors have no
glass at all. Instead, the coating is put on an alternative material.
The downside to first surface mirrors is that there is no glass to protect the metallic coating. The solution is usually an ultra-thin coating of silicon oxide (SiO) that enhances reflectivity and protects the metallic surface.
