|MadSci Network: Engineering
Most mirrors are made of two components: a flat, highly polished, shiny metal surface, and a flat piece of glass. The shiny metal does the real work, reflecting light rays back to the viewer. The glass simply serves to protect the metal; if it weren't there, the metal surface would tarnish, accumulate dust and fingerprints, and quickly lose its reflectivity. Ordinary mirrors use a flat piece of glass, mounted on top of the metal. Both the front and rear surfaces of glass are parallel to the shiny metal surface. A light ray which comes straight into the mirror, perpendicular to its surface, passes through the glass, bounces straight back off the metal, passes through the glass again, and comes out straight back towards its source. Rear-view mirrors in cars are made slightly differently: the glass cover is thicker at the bottom of the mirror than at the top. This means that the rear surface of the glass is parallel to the metal (since they touch each other), but the front surface of the glass is NOT. To use the mirror in "normal", or daytime mode, you adjust it so that the metal surface acts as the reflector. Light rays from a car behind you pass through the glass, strike the metal, bounce back, go through the glass again, and reach your eye. Like a regular mirror, the metal surface reflects most of the light which strikes it. During the daytime, this is good. But at night, the bright headlights of the car behind you may be so bright that they blind you. That's bad. It would be nice to have a mirror which reflected only a small fraction of the light striking it -- so that the bright headlights would appear to be dim headlights: still obvious enough to catch your attention, but not so bright that they blind you. To use the mirror in "dimmed", or nighttime mode, you adjust it so that the FRONT SURFACE of the glass acts as the reflector -- not the metal surface. Because glass is nearly transparent, only a small fraction of the light which strikes it will bounce back off it. You can detect this yourself sometime: stand in a room at night with all the lights on, and look out a window. You will see reflections of the lights in the window. Now, if you adjust the mirror so that light from the headlights of the car behind you bounce off the front of the glass at just the right angle to reach your eyes ... then the light rays which go through the glass and strike the metal surface will MISS your eyes. They will be reflected down farther, onto your mouth, or neck, or chest. That means most of the blindingly bright light rays won't enter your eyes, which is a good thing.
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