# Why are convex mirrors used instead of concave mirrors as rear view mirrors?

Oct 31, 2015

Convex mirrors always produce images that are virtual, upright and diminished.

#### Explanation:

According to the spherical mirror equation,

$\frac{1}{u} + \frac{1}{v} = \frac{2}{R}$,

Since $u$ is positive and $R$ is negative, $v$ will always be negative, and smaller in magnitude than $u$. Therefore the image will always be upright (not upside-down), virtual and diminished.

In the case of concave mirror, images of objects further from the focal point ($\frac{R}{2}$ from the mirror) will be inverted, which is no good to a driver. For objects nearer than the focal point, the image would be magnified. Given the small area of the rear, less images can fit in it. But more importantly, near objects may not be of little interest to drivers, and concave mirrors are not suitable to view far objects.

For convex mirrors, diminished images means that images appears smaller than the objects, which means that a larger field of vision could be obtained. This is also why a convex mirror is preferred over a planar one.

In summary, the driver needs a wide view of the road situation that are (relatively) far away, and convex mirrors are able to acheive that.