A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

How do "night" rear view mirrors work?

October 13, 1989

Dear Cecil:

How do "night" rear view mirrors work? One flick of the button and it seemingly dims all.

Dear Chris:

Here at Straight Dope University, we have explanations suited to all levels of intellectual attainment. We offer the intro-level course first.

In a dimming rear view mirror you've got two reflecting surfaces--one with high reflectance, one with low. During the day you use the high reflector. At night the dimmer button swings the low reflector into place, dimming glare from headlights behind you. Satisfied? Then cut to the funnies, wimp. Still thirsting for more? Coming right up.

The trick is that the two reflecting surfaces are the front and back of the same piece of glass. Said glass is specially ground so that the back surface is slightly tilted relative to the front one. In other words, the glass looks wedge-shaped from the side. The back surface is coated with silver like a bathroom mirror, making it highly reflective. The front surface isn't coated, but it's still slightly reflective, like all glass.

Because the two surfaces are out of parallel, any time you look at the rear-view mirror, you're seeing two different reflections simultaneously. During the day with the mirror tilted into the normal position, the silvered surface shows you the road behind you. The non-silvered surface, meanwhile, shows you the back seat of the car--but it's so dim you don't notice it.

At night the situation is reversed. When you flip the dim button, the silvered surface tilts so it's showing you the car's ceiling, which is so dark you don't notice it. But now the non-silvered surface is showing you the road.

Because the headlights of the cars behind you are so bright, the non-silvered surface reflects enough light to let you see what's behind you. But it's not so bright that you're blinded.

The folks at GM tell me that on Cadillacs you can now get a high-tech "electrochromic" mirror that dims at night automatically, without having to flip a switch. The Caddy mirror has only one reflective surface, but there's a special film in front of it that gradually darkens at night through the magic of electronics.

Very impressive. But for sheer low-tech genius the tilting surfaces are hard to beat.

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A Straight Dope Classic by Cecil Adams
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