You’re driving at night when, suddenly, you’re blinded by a piercing light from somewhere above. Are you about to be abducted by aliens!? Alas, no. A car has simply driven up behind you, and its headlights–reflected in your rear-view mirror–are dazzlingly bright. You flick the little tab beneath your mirror and the reflected headlights dim. Have you ever wondered how your rear-view mirror does this?
Actually, your rear-view mirror is not just one mirror – it has two reflective surfaces. At the back is a regular, shiny mirror. Right in front of this primary mirror, however, is a thin, glass wedge which reflects only about four percent of the incoming light.
Imagine holding a plate of glass in front of a regular mirror – you’d still see your reflection in the mirror, but you’d also see a fainter reflection from the plate of glass. In a rear-view mirror, this glass surface is a wedge, pointing somewhat downward.
With the mirror in its normal position, the glass surface points down into the driver’s seat. During the day you don’t notice this faint reflection of your lap because the reflection from the main mirror–pointing out the rear window–is so much brighter.
When you flip the switch at night, you change the angle of the whole setup. Now the primary surface is pointing up at the dark ceiling of your car, and the glass wedge points out the rear window. You can still see the headlights behind you, but since the glass wedge only reflects a fraction of the light, they appear much dimmer.