Ask A Scientist© Environmental Earth Science Archive

 
   name        Dave
   status      student
   age         13

   Question -
Glare from headlights can create serious vision problems for night
drivers. There is a feature of the rear view mirror that can be used to
prevent the glare of the headlights from reaching the driver's eyes. How
does it work?



        Rear view mirrors are made out of a mirror that is a wedge of glass
instead of two parallel planes of glass.

Rear View Mirror                Regular Mirror

I-----/                             I  I
I    /                              I  I
I   /                               I  I
I  /                                I  I
I /                                 I  I
I/                                  I  I


During daytime viewing, the rear view mirror is set so that the reflection
is from the back, silvered part of the mirror.  At night, the mirror is
tilted so that the reflection of the headlights behind you only hit the
front glass surface of the mirror and reflects from it.  Since this surface
is only 5% reflective, the glare is significantly reduced.  If you look at
the ceiling while someone else is driving using the night setting of the
mirror at night, you see a bright reflection from the car behind you.

        Since the night setting of the rear view mirror is a reflection from the
surface of the mirror's glass, such an arrangement would not work for
oncoming traffic.

---Nathan A. Unterman

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The front surface of the glass is not parallel to the back (silvered) surface.

Tim Mooney


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