A typical household mirror is a sheet of flat glass with a thin coating of metal on the back, usually aluminum or silver, sealed behind protective paint. The glass itself is not what reflects your image; it simply serves as a smooth, rigid support for the metallic layer that bounces light back to your eyes. But mirrors come in far more varieties than the one hanging in your bathroom, and the choice of materials shifts dramatically depending on whether the mirror needs to reflect visible light, infrared radiation, ultraviolet wavelengths, or laser pulses. The story of what a mirror is “made of” stretches from volcanic rock to nanometer-thick stacks of exotic ceramics.
Glass and Metal in an Everyday Mirror
The mirror on your wall has three basic layers. The front is a pane of soda-lime glass, the same type used in windows. Behind the glass sits an extremely thin film of reflective metal, typically deposited through a chemical or vacuum process. Behind the metal sits a coat of protective paint, often copper-tinted, that shields the fragile metallic layer from moisture and scratches. When light enters through the glass, hits the metal, and bounces back out, you see a reflection.
Soda-lime glass dominates because it is cheap, easy to manufacture in large flat sheets, and optically clear enough for everyday use. The glass does absorb a small fraction of the light passing through it, which is one reason bathroom mirrors are not quite as bright as a polished metal surface viewed head-on. For most people, that slight loss is invisible. The real work happens at the metal layer.
Why Silver and Aluminum Are the Go-To Metals
Silver has the highest reflectance of any metal across the visible spectrum, which is why it was historically the standard for high-quality mirrors. A freshly deposited silver coating reflects roughly 95 to 99 percent of visible light. The drawback is that silver tarnishes. Exposure to sulfur compounds in the air produces a dark silver sulfide layer that degrades the reflection over time. That dark spotting you see on antique mirrors is usually tarnished silver.
Aluminum replaced silver in many commercial mirrors during the twentieth century because it is more resistant to corrosion and far cheaper. A protected aluminum mirror, often sealed with a thin layer of silicon dioxide, achieves reflectance well above 90 percent in the visible range and performs reasonably well into the ultraviolet, though it drops below 90 percent at shorter UV wavelengths. Compared with silver mirrors, aluminum mirrors have a lower tendency to tarnish when exposed to humidity.1RP Photonics Encyclopedia. Metal-coated Mirrors For a bathroom mirror, that durability matters more than squeezing out another few percent of reflectance.
Protected silver coatings, also typically sealed with silicon dioxide, are preferred when performance matters more than cost. They work well from about 500 nanometers (green-yellow light) all the way out to 20 micrometers in the infrared. Their lower reflection losses also mean they can handle higher-powered laser beams before sustaining damage, which is why silver-coated optics show up frequently in laser systems.1RP Photonics Encyclopedia. Metal-coated Mirrors
Gold Mirrors and Infrared Applications
Gold might seem like an odd choice for a mirror, but it is excellent at reflecting infrared light. A protected gold mirror can achieve an average reflectance of about 97 percent between 700 and 2,000 nanometers, and in some infrared bands it pushes above 99 percent.1RP Photonics Encyclopedia. Metal-coated Mirrors The tradeoff is that gold is nearly useless below about 600 nanometers. It absorbs blue and green wavelengths, which is why gold looks golden to the eye rather than silver-white. That same property makes gold a poor choice for visible-light mirrors but ideal for telescopes and sensors that need to capture heat signatures or study distant, red-shifted astronomical objects.
The James Webb Space Telescope, for instance, uses gold-coated beryllium mirror segments specifically because its science targets emit infrared light. The beryllium underneath provides a lightweight, thermally stable structure, while the gold coating does the actual reflecting. That pairing illustrates a broader principle in mirror design: the substrate and the reflective coating are chosen independently, each optimized for a different job.
What the Mirror Sits On Matters Too
In a household mirror, the glass substrate just needs to be flat and cheap. In precision optics, the substrate can be the most expensive and carefully engineered part of the whole mirror. Telescope mirrors, for example, face a problem that bathroom mirrors never encounter: temperature changes cause materials to expand and contract, warping the mirror surface by amounts that are tiny in everyday terms but catastrophic for astronomical imaging.
One widely used solution is a glass-ceramic called ZERODUR, manufactured by Schott. It has an extremely low coefficient of thermal expansion, meaning its shape barely changes when the temperature shifts. Most of the major modern optical telescopes in the four-meter class and in the eight- to ten-meter class use ZERODUR mirror substrates.2Proceedings of SPIE. Forty years of ZERODUR mirror substrates for astronomy: review and outlook The material also has excellent dimensional stability, which matters when a mirror needs to hold its shape to within fractions of a wavelength of light over years of operation.3PubMed Central. A Study on the Mechanism of Selective Removal of ZERODUR Microcrystalline Glass by Polishing Abrasives in Magnetorheological Machining
Other substrates used in high-end optics include fused silica, borosilicate glass, silicon carbide, and even single-crystal silicon. Each has its own combination of weight, stiffness, thermal behavior, and cost. The choice depends on whether the mirror will fly on a satellite, sit in a mountaintop observatory, or ride inside a compact laser system.
Before Glass There Was Obsidian
Mirrors predate glass manufacturing by thousands of years. The earliest known mirrors were made of obsidian, a naturally occurring volcanic glass. Convex mirrors cut from obsidian have been found at the ancient settlement of Çatalhöyük in modern-day Turkey, dating to roughly 7400 to 6200 BCE.4Mediterranean Archaeology and Archaeometry (MAA). LOOKING THROUGH A TELESCOPE WITH AN OBSIDIAN MIRROR. COULD SPECIALISTS OF ANCIENT CULTURES HAVE BEEN ABLE TO VIEW THE NIGHT SKY USING SUCH AN INSTRUMENT? These were polished by hand until the surface was smooth enough to produce a recognizable reflection.
Obsidian mirrors are nowhere near as reflective as modern glass-backed metal mirrors. Their reflectance is low enough that when researchers built a small telescope with a 12-centimeter obsidian mirror, the instrument could show the Moon’s craters and the phases of Venus, but Jupiter’s moons were not visible because too little light was reflected.4Mediterranean Archaeology and Archaeometry (MAA). LOOKING THROUGH A TELESCOPE WITH AN OBSIDIAN MIRROR. COULD SPECIALISTS OF ANCIENT CULTURES HAVE BEEN ABLE TO VIEW THE NIGHT SKY USING SUCH AN INSTRUMENT? Still, for early civilizations, a polished piece of volcanic glass was a remarkable technology.
After obsidian came polished metal mirrors made of bronze, copper, and eventually speculum metal, a copper-tin alloy. These were the standard for centuries and even served as the primary mirrors in early reflecting telescopes. The shift to glass-backed silver happened in the nineteenth century, when chemists developed reliable ways to deposit a thin, even layer of silver onto a glass surface. That technique, broadly called silvering, opened the door to the mirrors we recognize today.
Dielectric Mirrors and Extreme Reflectance
Metal coatings top out at around 99 percent reflectance in practice, which sounds impressive until you need a mirror that wastes almost no light at all. Dielectric mirrors fill that gap. Instead of a metal layer, they use a stack of many thin transparent layers, each made of a different material with a different refractive index. When light hits each layer boundary, a small amount reflects. By choosing the layer thicknesses precisely, all those tiny reflections add up constructively at a target wavelength, producing reflectance that often exceeds 99.9 percent.5RP Photonics Encyclopedia. Dielectric Mirrors
The catch is that this constructive interference only works over a narrow band of wavelengths. A dielectric mirror designed to reflect 99.99 percent of 1,064-nanometer laser light might be mediocre at reflecting 800-nanometer light. That makes dielectric mirrors specialists rather than generalists. You will find them inside laser cavities, where even a fraction of a percent of lost light per bounce adds up over thousands of round trips, and in telecommunications equipment, scientific instruments, and anywhere else extreme reflectance at a specific wavelength justifies the cost.
The materials in these layer stacks are typically metal oxides and fluorides: titanium dioxide, silicon dioxide, hafnium oxide, magnesium fluoride, and others chosen for their optical properties at the wavelength of interest. The layers are deposited using vacuum techniques such as electron-beam evaporation or ion-beam sputtering, building up a coating that can contain dozens or even hundreds of individual layers.
Multilayer Mirrors for X-Rays and Extreme Ultraviolet
Normal mirrors, whether metal-coated or dielectric, do not work well at very short wavelengths like X-rays and extreme ultraviolet light. At those energies, most materials absorb rather than reflect. Over the past several decades, researchers have developed multilayer structures specifically for this regime, using physical vapor deposition to lay down alternating films of two materials whose optical properties are suited to extremely short wavelengths.6Topical Meeting on Free Electron Generation of Extreme Ultraviolet Coherent Radiation. Multilayer Mirrors and Optics for Low Energy X-Ray and Extreme Ultraviolet Regimes Common pairings include molybdenum and silicon for extreme ultraviolet, and tungsten and carbon for certain X-ray wavelengths.
These mirrors are essential in semiconductor lithography, where extreme ultraviolet light is used to print circuit features smaller than 10 nanometers onto silicon wafers. Each mirror in the optical path reflects only a portion of the incoming light, so the mirrors need to be as efficient as possible to keep enough photons reaching the wafer. They also appear in synchrotron beamlines and space-based solar telescopes. The materials are exotic compared to a bathroom mirror, but the underlying idea is the same: choose the reflective material to match the wavelength you need to bounce.
Why Mirrors Degrade Over Time
If you have ever seen an old mirror with dark blotches creeping in from the edges, you have seen what happens when the protective layers fail. Silver is particularly vulnerable to sulfur-containing gases, which react with the metal to form silver sulfide, a dark compound that kills reflectance. Copper, often used as a secondary protective layer between the silver and the backing paint, can also corrode, forming copper sulfide and copper oxide. Studies examining tarnished silver and copper surfaces have confirmed that sulfide formation is the primary chemical culprit behind mirror degradation.7PubMed Central. A dataset of high-resolution synchrotron x-ray photoelectron spectra of tarnished silver-copper surfaces before and after reduction with a remote helium plasma at atmospheric pressure
Humidity accelerates the process. Bathrooms, with their steam and temperature swings, are especially hard on mirrors. High-end mirrors use better edge sealing and additional protective coatings to slow this down, but no silver-backed mirror lasts forever in a wet environment. Aluminum-backed mirrors resist this kind of corrosion better, which is one more reason they dominate the mass market.
For scientific mirrors, degradation is an even bigger concern because even a tiny drop in reflectance can affect measurements. Observatory mirrors are periodically re-coated by stripping the old metal layer and depositing a fresh one, a process that typically takes the telescope offline for days. Some facilities re-coat their primary mirrors every few years to maintain peak performance.
Electrochromic and Smart Mirrors
A newer category of mirror technology does not just reflect light passively but can actively change how much light it transmits or reflects. Electrochromic devices switch between transparent and opaque states when a voltage is applied. You may have encountered them in auto-dimming rearview mirrors, which darken automatically when headlights from behind would otherwise blind you. The same technology is being developed for smart windows, automotive sunroofs, and architectural glass.8ECS Meeting Abstracts. Optimizing the Temperature Stability of Roll-to-Roll-Processable Polymeric Electrochromic Devices for Smart Window Applications
The materials in an electrochromic mirror typically include a thin-film electrode made of a metal oxide like tungsten trioxide, an electrolyte layer, and a counter-electrode. When current flows, ions shuttle between the electrodes, changing the optical properties of the active layer. Polymeric versions of these devices are particularly promising because they can be manufactured using roll-to-roll processing, similar to how newspapers are printed, which could make them cheap enough for widespread consumer use.
Metasurface Mirrors and Nanophotonic Design
At the frontier of mirror technology, researchers are building reflective surfaces from arrays of nanostructures rather than continuous metal films. These metasurfaces consist of tiny engineered elements, smaller than the wavelength of light, arranged on a flat surface. By controlling the shape, size, and spacing of these elements, designers can manipulate reflected light in ways that a simple metal coating cannot, including focusing it, steering it at angles, or selectively reflecting only certain polarizations.9PubMed Central. Recent Advancement in Optical Metasurface: Fundament to Application
One example uses amorphous titanium dioxide nanofins sitting on a silver mirror substrate. Titanium dioxide was chosen because it has a high refractive index and very low light absorption at visible wavelengths, and the nanofin structures are small enough to control the wavefront of visible light while remaining manufacturable with standard nanofabrication tools.10Scientific Reports. Visible light focusing flat lenses based on hybrid dielectric-metal metasurface reflector-arrays The result is a flat surface that can focus light like a curved mirror would, but without any curvature. These hybrid dielectric-metal designs combine the high broadband reflectance of a metal film with the wavefront-shaping precision of nanostructured dielectrics.
Metasurface mirrors are still largely confined to laboratories and specialized applications. Making them cheaply at large scales remains a challenge. But they point toward a future where a “mirror” is not just a passive reflective surface but a designed optical component that controls light in programmable, application-specific ways. The materials in such a mirror might include silicon, germanium, gallium arsenide, or various metal oxides, depending on the operating wavelength and function. The concept of what a mirror is “made of” keeps expanding as fabrication techniques reach finer and finer scales.