Understanding Mirrors: Reflection, Uses & Image Formation

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Mirror

Mirror, a smooth, polished surface that reflects light rays and causes images to be formed. Mirrors can be used to view objects that are out of the line of sight, to increase illumination, to focus light or heat, or to form enlarged or reduced images. In everyday life, mirrors are used for such things as looking glasses and headlight reflectors. Mirrors are used in such instruments as microscopes and telescopes.

How Images Are Formed By Mirrors

The angle at which light is reflected from a surface (called the angle of reflection) is equal to the angle at which light strikes the surface (angle of incidence), as shown in the drawing on the facing page. Most surfaces are irregular, and light striking them is diffusely reflected (scattered). Since a mirror has an extremely smooth surface, light is evenly reflected from it. This type of reflection, called regular reflection, makes possible the image seen in mirrors.

The image formed by a mirror can be real or virtual. A real image is inverted (upside down), appears in front of the mirror, and may be larger or smaller than the object. A real image may be projected onto a screen. A virtual image is erect (upright), appears to be behind the mirror, and may be larger or smaller than the object. A virtual image in a mirror has the sides reversed; that is, the left side of the object will appear as the right side of the mirror image. The image in a looking glass is a virtual image. A virtual image cannot be projected.

Types of Mirrors

Mirrors may be flat or curved. A flat, or plane, mirror causes a minimum of distortion. It produces a virtual image that appears as far behind the mirror as the object is in front. For a person to see himself completely in a plane mirror, the mirror must be at least half as tall as he is.

Curved mirrors may be either convex (curved outward) or concave (curved inward). A convex mirror, like a plane mirror, produces only virtual images. The images are always smaller than the objects being reflected. A concave mirror may produce either a real image or a virtual image, depending on the distance between the object and the mirror.

If a concave mirror is shaped like part of a sphere, the real image that it forms will be distorted. This distortion, called spherical aberration, can be avoided by using a mirror of a different shape, such as a parabolic mirror.

How Mirrors Are Made

Most mirrors are made by coating a piece of glass with a layer of metal. The glass provides the smooth surface necessary for good reflection and protects the metal from becoming scratched and tarnished. Polished plate glass is used for the finest mirrors. A solution containing silver nitrate is poured onto the glass, and a deposit of silver adheres to the glass. The remains of the solution are poured off and the glass is dried. The silvered surface (the back of the mirror) is then coated with a protective substance, such as shellac, to keep the silver from being rubbed off or scratched.

In optical instruments such as telescopes, the front side of the mirror is silvered instead of the back. This is done to prevent the distortion that occurs when light rays are refracted, or bent, as they enter and leave the glass. Materials other than silver are used to reflect particular types of light. For example, aluminum is better than silver for reflecting ultraviolet light; gold is better for infrared light.

A one-way mirror is made by putting a very thin layer of silver on the glass, permitting some light to pass through. The mirror looks like an ordinary looking glass when viewed from a brightly lit area, but is transparent when viewed from a darkened area.

History of Mirrors

In ancient times, mirrors were made from polished metal. Bronze mirrors were used in Greece, Egypt, and Rome. The art of making flat glass mirrors originated in the Middle Ages. It was known in Nuremberg and in Venice in the 14th century. The Venetians soon became leaders in mirror production because of the superior quality of their work. Venetian mirrors were made of blown-glass cylinders that were split and laid out into flat sheets. The back of the glass was coated with an amalgam of tin and mercury. In the 17th century a group of Venetian craftsmen became established in France.

The technique of silvering mirrors using silver nitrate was discovered in the mid-19th century by Justus von Liebig, a German chemist.