What is a Convex Mirror?

Convex mirrors are also often called diverging mirrors, fisheye mirrors, wide angle mirrors or dome mirrors. They are curved mirrors featuring a reflective surface that bulges toward the light source. This means that light reflected from the curved surface spreads out or diverges.

What Does Convex Mean?

The term ‘convex’ simply means that a surface is curved outwards. Examples of convex surfaces include balls of all shapes and sizes (including our eyeballs), the back of spoons and magnifying glasses.

The opposite of convex is concave. Concave describes a shape that curves inwards, like a spoon or the inside of a bowl.

Characteristics of Convex Mirrors

A convex mirror can be formed from a section of the curved surface of a sphere. They are called diverging mirrors due to how incident light origination from a single point will reflect off the mirror surface and diverge.

Image: Wikipedia.

When a convex mirror is formed from a sliced portion of a sphere the centre of that sphere is known as the centre of curvature. A line that passes from the mirrors surface through the centre of curvature is known as the principal axis.

The convex mirror has a focal point, F, that’s located midway between the mirror surface and the centre of curvature (2F). Both the centre of curvature and the focal point are located behind the convex mirror. This means the mirror has a negative focal length value.

Since the light reflected from a convex mirror diverges the reflected image presented in always virtual and located behind the mirror. A virtual image means it cannot be displayed on a screen, but it can be seen. The reflected image of an object is always the right way up and will be diminished in size due to the curvature of the mirror.

This is the principle that provides the wide-angle or fisheye reflections in convex mirrors.

What is the Vertex of a Convex Mirror?

The convex mirror vertex is the point on the surface of a convex mirror where the principle axis meets the mirror.

Who Invented the Convex Mirror?

Convex mirrors have been known by many names including butler’s mirrors, bankers mirrors and the witch’s eye.

The very first glass mirrors are known to have been used in the Neolithic period in Turkey, Central America and other locations. They were made from obsidian, a naturally occurring black glass commonly derived from volcanic activity. When polished this reflective material provides a mirror surface.

The ability to make high quality glass was developed by the Romans but subsequent to the fall of the Roman empire much of this knowledge and these skills significantly declined. By the fourteenth century glass production in the west had resurfaced and convex mirrors made from relatively clear glass were being made in a variety of European locations. An example can be seen in Jan Van Eyck’s Wedding Portrait (1434), in which a convex mirror is centrally located on a wall behind the subjects, providing a distorted view of them and the room they were in.

The Wedding Portrait by Jan Van Eyck, 1434.

Where are Convex Mirrors Used?

Convex mirrors have a multitude of applications across all sectors including industrial, commercial, architectural and more.

We are all familiar with convex mirrors used in road safety applications where they provide drivers and road users with an expanded, wide-angle view. This beneficially overcomes visibility issues, enabling drivers and others to effectively see around corners.

Driveway safety mirror in hedgerow. Image: Insight Security.

Another common application we are all familiar with is convex security mirrors used in shops. These provide shop workers with a convenient and safe way to monitor shop visitors from their checkout positions.

Convex Retail Security Mirror. Image: Insight Security

Convex mirrors are also used in automobile rear view and side mirrors as they provide a wide angle, expansive view.

Another common application for convex mirrors is in street light reflectors. Since the convex mirror surface diverges light it serves to expand the area illuminated by the street light.

And convex mirrors are used for inspection purposes. For example, examining the underside of vehicles is often carried out with the aid of convex mirrors. Also, customs and excise officers use convex mirrors in their search and inspection operations.

Convex mirrors are widely used in prisons, custody centres and other institutions where they aid safety and security.

Anti-ligature and institutional convex mirrors. Image: Insight Security

Convex mirrors are also used in wide variety of industrial and commercial applications, from warehouses to factories, foundries and much more. They improve safety by enabling people, such as forklift drivers, to see around corners.

Warehouse and Industrial Convex Mirrors. Image: Insight Security.

Convex mirrors are also used at ATM machines where they allow users to check who is behind them and whether they are attempting to observe their PIN entry.

As noted, convex mirrors are used in a wide variety of practical applications and they’ve also been popular as decorative mirrors in homes for centuries, demonstrated in this piece of artwork by Netherlands artist Juan d Flandes from the 16th century.

The Marriage Feast at Cana by Juan de Flandes (artist, Netherlandish, active by 1496–died 1519 Palencia)

Why are Vehicle Mirrors Slightly Curved Convex?

One of the questions arising in UK driving theory test is: Why are vehicle mirrors slightly curved (convex)? The correct option from the multiple choice answers is: They give a wider field of vision.

Driving theory test question – the right answer.

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