Mirrors reflect light because their surfaces are designed to produce strong specular reflection, in which incoming light rays are redirected in an organised manner rather than scattered randomly in many directions. Reflection occurs when electromagnetic radiation encounters a material boundary and the electric field of the light interacts with charges in the material. At a conventional household mirror, a thin reflective metal layer, commonly aluminium or silver, is deposited behind or sometimes in front of a protective transparent layer.
The metal contains mobile electrons that respond strongly to the electric field of incident light and re radiate electromagnetic energy. The combined electromagnetic response produces reflected light that travels away from the surface according to the law of reflection. The angle at which the light approaches the surface and the angle at which it leaves are measured relative to the normal, an imaginary line perpendicular to the surface at the point of incidence. For an ideal flat mirror, these two angles are equal.
The law of reflection is central to understanding how a mirror forms images. If a light ray approaches a flat mirror at an angle of incidence, the reflected ray leaves at an equal angle on the opposite side of the normal. The reflected rays remain ordered because the mirror surface is sufficiently smooth on the relevant scale. A rough surface also reflects light, but microscopic irregularities cause the reflected rays to travel in many different directions, producing diffuse reflection.
This distinction explains why a painted wall does not normally form a clear image while a polished mirror does. Both surfaces reflect light, but their surface structures produce very different angular distributions of reflected radiation. The smoothness required for mirror like reflection depends on the wavelength of the radiation being considered. A surface that appears smooth to visible light may behave differently for much shorter or longer wavelengths. Thus mirror reflection is not an absolute property of a material independent of wavelength and surface structure.
A plane mirror produces a virtual image because reflected rays entering the observer's eyes diverge as though they originated from a location behind the mirror. The image is called virtual because the light does not actually converge at the apparent image position. If the reflected rays are extended mathematically backwards, they appear to meet at corresponding points behind the reflecting surface.
The image formed by an ideal plane mirror has the same size as the object and is located the same perpendicular distance behind the mirror as the object is in front of it. It is upright relative to the object but appears laterally reversed in ordinary descriptions. More precisely, a mirror reverses the direction perpendicular to its surface while preserving the directions parallel to the surface. The common statement that a mirror reverses left and right is therefore a simplification. If a person turns around to face the opposite direction, their physical left and right relationships change relative to the mirror, which contributes to the familiar appearance of lateral reversal.
Mirrors are usually made from a transparent substrate such as glass combined with a reflective coating because the glass provides mechanical support and protection for the reflective layer. Modern mirrors may use aluminium or silver because these metals reflect a large fraction of visible light across useful wavelength ranges. The exact reflectance depends on wavelength, angle of incidence, surface quality, coating thickness, and material composition. A mirror does not reflect one hundred percent of incident visible light.
Some light can be absorbed within the coating, some can be transmitted through it, and some can be lost through imperfections or scattering. The glass itself can also introduce reflection and transmission at its surfaces. Household mirrors commonly place the metal layer behind the glass, protecting it from physical damage and oxidation. Front surface mirrors place the reflective coating at the front and are useful in optical instruments because they reduce unwanted reflection and refraction caused by the glass substrate.
Curved mirrors use the same reflection principle but alter the paths of reflected rays because the surface normal changes from point to point. A concave mirror curves inward and can cause parallel incoming rays to converge towards a focal region, while a convex mirror curves outward and causes reflected rays to diverge as though they originated from a virtual focal point behind the mirror.
The location, orientation, and size of an image depend on the object's position relative to the mirror's focal length and centre of curvature. Concave mirrors can form either real or virtual images depending on object position. Convex mirrors form upright, reduced virtual images for ordinary objects. These properties are exploited in vehicle mirrors, security mirrors, reflecting telescopes, searchlights, optical instruments, and solar concentrators. The geometry can be analysed using ray diagrams and the mirror equation, although real optical systems require consideration of aberrations and finite aperture.
Reflection from mirrors is not limited to visible light. Reflective surfaces can operate across many parts of the electromagnetic spectrum, although the materials and structures needed for efficient reflection can differ substantially with wavelength. Radio astronomy uses highly reflective metal dishes to redirect radio waves towards receiving systems.
X ray telescopes cannot normally use ordinary mirrors at normal incidence because X rays penetrate or are absorbed by many materials, so specialised grazing incidence mirror geometries are used to redirect X rays at very shallow angles. Infrared instruments can also use reflective optics because mirrors can avoid some chromatic effects associated with refractive lenses. The underlying electromagnetic principle remains reflection, but the engineering requirements depend strongly on wavelength. Surface roughness, coating properties, thermal expansion, contamination, and alignment become increasingly important in precision optical systems.
Mirrors have numerous practical applications because they can redirect light without requiring it to pass through a transparent material. Periscopes use arrangements of mirrors or prisms to allow an observer to see around or above obstacles. Reflectors in vehicle lamps direct light into useful beams. Optical instruments use mirrors to fold light paths, control beam direction, or form images.
Large astronomical telescopes frequently use mirrors rather than large lenses because mirrors can be supported from behind and do not need to transmit light through the entire thickness of the optical element. This allows very large apertures to be engineered more practically. Mirrors are also important in interferometers, laser systems, scanning devices, communications equipment, and scientific instruments. In everyday life, mirrors are primarily used for visual observation, but the underlying technology is part of a much broader field of optical engineering.
The key distinction between reflection and refraction is that reflection redirects light back into the original medium, whereas refraction involves a change in direction as light passes between media with different optical properties. At a mirror, the incident and reflected rays remain on the same side of the interface in the usual geometric description. Some materials can both reflect and transmit light, and at any real boundary the amount of reflection depends on the refractive indices and angle of incidence.
At sufficiently large angles, certain transparent materials can exhibit total internal reflection, which is not the same mechanism as metallic mirror reflection even though both can redirect light efficiently. The behaviour of mirrors therefore arises from fundamental electromagnetic interactions at material surfaces. A household mirror is a familiar example of a highly engineered optical surface that converts the microscopic response of electrons to electromagnetic radiation into a macroscopic image that can be predicted using geometric optics.
