There are areas of the road, behind the car and to the sides, which are crucial to be able to see, in order to drive safely, which are out of the driver's peripheral vision. The rear view mirror on a car is an interior mirror which allow the driver to see the 'blind spots' behind the car, enabling clear vision of any traffic nearby and any potential hazards, and as such is an important visual aid and safety feature.
The rear view mirror is housed in a casing and mounted centrally, usually to the top of the front windshield on a double-swivel mount which allows it to be adjusted vertically, horizontally and diagonally to suit the height, line of sight and seat position of the driver. Internal rear view mirrors can be operated either electrically or manually.
Rear view mirror glass comes in a variety of options, such as plain glass, convex, or prismatic. Prismatic rear view mirrors are operated by hand and can be tilted using a tab below the mirror. This enables the driver to adjust the mirror to suit day or night driving. There is a clever automatic dimming feature available on some electric models which does this for you and reduces the glare in the rear view mirror from cars travelling behind. The rear view mirror glass should be clean and free from scratches, chips and cracks, to enable clear, safe vision. Rear view mirrors come in different shapes and sizes, dependent on the make and model of car, so it's important to ensure that you replace the rear view mirror with a part that is compatible with your vehicle.
Broken, smashed or chipped rear view mirrors will, of course, need replacing. Over a period of general use, the rear view mirror will suffer from general wear and need replacing with a rear view mirror that is compatible with the make and model of your car.