Because of its curvature. Because they are curved, car mirrors give the driver a greater field of vision than the common mirror. This is because the size of a mirror’s reflection is determined by its curvature. Despite looking flat, car mirrors have what physicists call a «convex spherical shape». Just imagine that they are a small piece of a very large circle. “Compared to the flat mirror, the convex mirror forms smaller images than the original object, but increases the observed region”, says physicist João José Caluzi, from the São Paulo State University (Unesp) in Bauru (SP). The invention of this indispensable artifact for any car is credited to the Frenchwoman Davy de Cussé. In 1897 – a time when cars barely exceeded 40 km/h -, she decided to put a mirror in the front of the car to see who was behind. The girl’s mirror, however, was flat and had a reflection just like the one in the bathroom at home. Convex mirrors only reached automobiles almost a century later, in 1970. The use of this type of mirror was a safety determination by the American government. Quickly, this fashion for good was adopted in all countries of the world.
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Vision beyond reach Rear view mirrors are curved and produce a much larger reflection than common mirrors
COMMON MIRROR
In the common mirror — the plane —, the distance of the reflection from the object is the same as the real object is from the mirror. As light rays return at the same angle, they hit the eye the same way they would if they were reflected by the object itself.
REARVIEW MIRROR
In the rearview mirrors – curved convexly, as you see in the drawing – the reflected object appears closer than the real object. This makes things smaller, but the field of view increases. In today’s traffic, that’s a big plus.