You’re driving at night when, suddenly!, you’re blinded by a piercing light from somewhere above. A car has 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.