Mirror

• 1. Introduction

  ›

  A mirror, at its simplest, is an object that reflects an image back to whoever stands before it. Yet that plain description conceals one of the oldest and most persistently strange relationships between human beings and their own faces. Pools of still water served as the earliest mirrors long before anyone shaped stone or smelted metal. By around 6000 BCE, people in Anatolia were already crafting mirrors from obsidian, a naturally occurring volcanic glass, at the site of Catalhoyuk. What drove them? What have we seen in these surfaces that has made us keep building them, improving them, and weaving them into art, religion, science, and superstition across every civilization we know? The answers take us from ancient Mesopotamia to the glass workshops of Venice, from the physics of electromagnetic waves to the nightmares of horror cinema, and deep into the unresolved question of why a mirror appears to reverse left and right but never top and bottom.

• 2. How Mirrors Actually Work

  ›

  Light consists of waves, and when those waves strike a flat mirror surface, they bounce back retaining the same degree of curvature they had on the way in, just directed in the opposite sense. This is the principle physicists call specular reflection, and it is what distinguishes a mirror from a coat of flat-white paint, which scatters light in every direction and destroys the wave pattern entirely. A surface qualifies as a mirror whenever its texture is smoother than the wavelength of the waves hitting it. Visible light has wavelengths of only a few hundred nanometers, a few hundred-thousandths of an inch, so glass and polished metal must be extraordinarily smooth to produce a clear reflection. Microwaves, by contrast, sometimes have wavelengths greater than an inch, which means they can reflect specularly off a metal screen door or even a continental ice sheet.

  A plane mirror produces an image that looks normal and undistorted, but curved mirrors behave differently. A concave parabolic mirror gathers parallel rays and bends them through a single focal point, making it ideal for telescopes and solar furnaces. A convex mirror spreads those same rays outward, which is why parking lots and large trucks rely on convex rear-view mirrors to widen the driver's field of view.

  The famous puzzle of mirror reversal turns out to hinge on geometry and perception combined. A mirror does not actually swap left and right: it swaps front and back. When a person raises their left hand, the actual left hand rises in the reflection too. The illusion of reversal arises because the brain unconsciously imagines the reflected figure has turned around to face them, the same way a real person would have to turn in order to face them, and that imagined rotation is what introduces the apparent left-right swap. Text appears reversed in a mirror for the same reason: to read the text directly, one would have to physically rotate the paper to face oneself, and that rotation is what flips the letters.

• 3. Stone, Metal, and Glass

  ›

  Mirrors of polished copper were being made in Mesopotamia from around 4000 BCE, and in ancient Egypt from roughly 3000 BCE. Polished stone mirrors from Central and South America date from around 2000 BCE onward. By the Bronze Age, most cultures had settled on polished discs of bronze, copper, silver, or other metals, and the people of Kerma in Nubia were particularly skilled manufacturers; remains of their bronze kilns have been found inside the temple of Kerma itself.

  Bronze mirrors had low reflectivity and poor color rendering, and stone mirrors were worse still. These deficiencies are precisely what gave rise to the New Testament phrase in 1 Corinthians 13 about seeing as in a mirror, darkly. An alloy called speculum metal, a highly reflective mix of copper and tin, raised the standard somewhat and was still in use until a couple of centuries ago, though it may have originated in China and India.

  Glass began to enter mirror-making in the 1st century CE, alongside the development of soda-lime glass and glassblowing. The Roman scholar Pliny the Elder described artisans in Sidon, in what is now Lebanon, producing glass mirrors coated on the back with lead or gold leaf. The glass gave a smooth surface and shielded the metal from scratches, but these early examples were small, ranging from 10 to 20 cm in diameter, and often concave or convex rather than flat. Lead-coated versions had to be made very thin to prevent cracking from the heat of the molten metal. Despite their limitations, glass mirrors pointed toward the future. Silver-coated metal mirrors were developed in China as early as 500 CE, where the bare metal was coated with an amalgam and then heated until the mercury boiled away.

• 4. The Venetian Monopoly

  ›

  Glassmakers in France found a way to make flat glass plates by blowing glass bubbles, spinning them rapidly to flatten them, and cutting rectangles out of the resulting discs. A better technique emerged in Germany and was then refined in Venice by the 16th century: blow a cylinder of glass, cut off the ends, slice it along its length, and unroll it onto a flat hot plate. Venetian glassmakers also adopted lead glass for its crystal clarity and easier workability.

  During the early European Renaissance, a fire-gilding technique appeared in which the back of the glass was coated with a tin-mercury amalgam and then gently heated to evaporate the mercury. This caused less thermal shock than the older molten-lead method. By the 16th century Venice had become the center of this process, and the resulting mirrors could be as large as 40 inches square. Venice kept its monopoly on the tin amalgam technique for about a century. These Venetian mirrors, set in richly decorated frames, were prized luxury items for palaces across Europe. A single example of how expensive they were: in the late 17th century, the Countess de Fiesque reportedly traded an entire wheat farm for one mirror, and considered it a bargain.

  That monopoly eventually broke through industrial espionage. French workshops obtained the secret and succeeded in scaling the process to mass production, making mirrors affordable to ordinary households despite the acknowledged toxicity of mercury vapor. It was German chemist Justus von Liebig who took the next decisive step in 1835, inventing the silvered-glass mirror through a wet deposition process: a thin layer of metallic silver deposited onto glass via the chemical reduction of silver nitrate. This silvering technique was adapted for mass manufacturing and is the direct ancestor of how most mirrors are made today.

• 5. The Science of Coatings

  ›

  Silver has the highest reflectivity of any metal in the visible to near-infrared range, capable of reflecting up to 98 or 99 percent of light at wavelengths as long as 2000 nm. Its drawback is that it tarnishes quickly and is expensive. Aluminium reflects 85 to 90 percent of visible and near-ultraviolet light, is harder, and resists tarnishing better, though it loses reflectance in the 800 to 900 nm band. Gold does not tarnish at all and reflects more than 96 percent of near- and far-infrared light in the 800 to 12000 nm range, but it handles visible light poorly at wavelengths shorter than 600 nm.

  Dielectric mirrors, which use layers of transparent materials chosen for their refractive indices, can exceed 99.99 percent reflectivity, though only across a narrow band of wavelengths ranging from as little as 10 nm to as wide as 100 nm for tunable lasers. The first dielectric mirror was created in 1937 by Auwarter using evaporated rhodium. Also in 1937, Hass created the first metallic mirror enhanced with a dielectric coating of silicon dioxide. Two years later at the Schott Glass company, Walter Geffcken invented the first dielectric mirrors to use multilayer coatings.

  Evaporation coating as a technique traces to a phenomenon noticed with incandescent light bulbs: metal from the hot filament slowly sublimated and condensed on the inner surface of the glass. Pohl and Pringsheim developed this into a practical method in 1912. John D. Strong then applied it to make the first aluminium-coated telescope mirrors in the 1930s. The technique called sputtering grew from observations in the 1920s and 1930s that metal was being ejected from electrodes in gas discharge lamps and condensing on glass walls, and it became an industrial coating method with the rise of semiconductor technology in the 1970s.

• 6. Mirrors in Art and Culture

  ›

  Leonardo da Vinci called the mirror the master of painters. He advised that when a painter wished to judge whether a picture matched its subject, they should reflect the actual object in a mirror and compare the reflection with the painting, noting particularly whether both likenesses of the subject corresponded. Filippo Brunelleschi discovered linear perspective with the help of a mirror. M. C. Escher used specially shaped mirrors to achieve a more complete view of his surroundings than direct observation allowed, most notably in Hand with Reflecting Sphere from 1935.

  Some of the most celebrated paintings in European art place a mirror at their center. Jan van Eyck's Arnolfini Portrait, Diego Velazquez's Las Meninas, and Edouard Manet's A Bar at the Folies-Bergere, painted in 1882, all use the reflective surface to multiply viewpoints in ways that painting from a single perspective could not. The psychological effect known as the Venus effect describes a common convention in painting and film: a character is shown apparently gazing at their own reflection, yet the angle at which the mirror is placed means the viewer sees the character's face, not what the character would actually see in reality.

  In medieval Europe, mirrors carried heavy symbolic weight in addition to practical uses. Burgundian ducal inventories record that the dukes owned mirrors combined with reliquaries and religious paintings for use in private devotion. The famous Arnolfini Wedding by Jan van Eyck has been read as assembling a constellation of objects, including a mirror surrounded by scenes of the Passion, a rosary, and a cushioned bench serving as a prie-dieu, all of them aids to private prayer. Mary herself was sometimes figured as the speculum sine macula, Latin for mirror without blemish. The mirror's physical property of showing truth was borrowed as a moral metaphor: depicted mirrors reminded viewers to examine themselves and reflect on the state of their own souls.

  The Greek philosopher Socrates urged young people to look at themselves in mirrors so that those who were beautiful would become worthy of their beauty, and those who were not would know how to compensate through learning.

• 7. Mirrors at Work in the World

  ›

  A signal sent by mirror and sunlight can travel as far as 60 km on a clear day. Native American tribes and numerous militaries used this technique to relay information between distant outposts, and specialized daylight signaling mirrors are still included in military survival kits.

  Periscopes, which rely on mirrors to redirect a line of sight around obstacles, were deployed to great effect during the World Wars, letting soldiers observe over the parapet of trenches without exposing themselves to direct fire. Tradition holds that Archimedes used a large array of mirrors to set Roman ships alight during an attack on Syracuse, though this has never been confirmed. A team from the Massachusetts Institute of Technology attempted to recreate the experiment on the television program MythBusters, using bronze mirrors of the type available in Archimedes' time, and found that the time required to ignite a ship would have made the tactic impractical. The team concluded the legend was busted, but did note that the concentrated reflected light would have severely impaired the vision of sailors on the targeted vessel.

  The Italian town of Viganella, set in a steep-sided valley, receives no direct sunlight for seven weeks every winter. In 2006, a computer-controlled mirror measuring 8 by 5 meters was installed at a cost of 100,000 euros to redirect sunlight into the town's piazza. In 2013, a similar installation brought sunlight into the town square of Rjukan in Norway. Texas Instruments' DLP technology places a postage stamp-sized microchip containing millions of microscopic mirrors at the heart of many large high-definition televisions and video projectors; each tiny mirror either reflects light toward the screen or toward a light-absorbing surface to create individual pixels. Optical discs such as CDs and DVDs are essentially modified mirrors, encoding binary data as a series of pits and lands on an inner layer that distort a reflected laser beam in ways the reader can decode.