Why Convex Mirrors are Used as Rear View Mirrors

This question is often asked: Why are convex mirrors used as rear view mirrors?
Students are then typically presented with a number of ‘multiple choice’ answers from which they need to select the correct one.

The reason convex mirrors are used as rear view mirrors in automobiles is they provide drivers with a wider view of what’s behind them. Convex mirrors, also known as diverging mirrors, present an erect (right way up), virtual, small sized (diminished) image.

Real and Virtual Images

The difference between real and virtual images is often confusing. Images formed by mirrors and lenses can be:

  • Upright or inverted
  • Magnified or diminished
  • Real or virtual.

Real Images

Real images are defined as:

An image that is formed when the light rays from an object converge and meet each other and can be projected onto a screen.

An important attribute of real images is that they can be projected, for example onto a screen. Cinema screen images are a familiar example of real images.

Real images are formed by the convergence of light towards a focal point. Real images can be formed by concave mirrors and converging lenses. If an object is placed further away from a concave mirror than the focal point a real but inverted image is reflected. If the object is brought closer to the concave mirror, when it passes the focal point, where optical rays converge, the image becomes virtual and is no longer inverted.

A simple demonstration of this principle can be seen using a large shiny spoon.

The image reflected from the concave side of the spoon is inverted, diminished and real. But if the reflected object is brought closer to the concave surface when it passes the focal point the image becomes non-inverted and virtual.

Virtual Images

Virtual images are defined as:

An image that is formed when the light rays from an object do not meet but appear to meet behind the lens and cannot be projected onto a screen.

Convex mirrors and plane, flat mirrors form virtual images. Images reflected from flat mirrors are the same size as the object and appear to be the same distance behind the mirror as the object is from the front of the mirror.

Convex or diverging mirrors present virtual images that are always upright, can’t be projected onto a screen and are diminished in size.

The back or convex side of a reflective spoon demonstrates this principle.

Convex rear view mirrors in automobiles provide drivers with a wide-angle field of view behind their vehicle. The reflected images are smaller (diminished) than the reflected objects (vehicles) and the wide-angle view provides drivers with a more expansive view of what’s behind them , including vehicles that may be overtaking.

rear view image
Tagged ,

Leave a Reply