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What happens when you use the tab on an inside rearview mirror to diminish the brightness of the headlights behind you? RC, Vail Colorado

mirrorA rearview mirror actually produces two different reflections, one from its back surface and one from its front surface. Its back surface reflects nearly all of the light hitting it, producing the bright reflection that you use for daytime driving. In contrast, the front surface reflects only about 4% of the light that hits it, yielding the dim nighttime reflection that makes car headlights less dazzling. To allow you to choose which surface reflection you see, the mirror's glass is wedge-thicker at the top than at the bottom. Since the reflective surfaces aren't parallel to one another, their reflections travel in different directions. You choose which reflection you see by adjusting the tilt of the mirror, using the tab near the bottom of the mirror. To give the back surface its high reflectivity, the manufacturer coats it with aluminum. Since aluminum is an excellent conductor of electricity even at very high frequencies, it reflects the electromagnetic waves of visible light with nearly perfect efficiency. The light waves cause electric currents to flow in the aluminum and those currents in turn redirect the light waves so that they reflect back from the surface. However, the front surface of the glass is uncoated and its reflection occurs via a somewhat different process. As a light wave attempts to enter the glass, it encounters a change in its material environment and therefore a change in the way it propagates forward. Glass is different from air and the relationship between a light wave's electric and magnetic components is different in the two media. As it passes from one media to the other, a light wave must adjust to an abrupt change in this relationship, a change known generally as an impedance mismatch. To accommodate this impedance mismatch, about 4% of the light wave is unable to enter the glass. Instead it reflects back from the glass surface. This dim reflected light is what you see when your mirror is tilted to its nighttime setting.

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Answered by Louis A. Bloomfield of the University of Virginia