Mirrors appear silver in color because they are made with a thin layer of silver or aluminum deposited on the back of a piece of glass. The silvered backing reflects light, allowing us to see our reflection. But why silver? Why do mirrors use silver or aluminum rather than other metals or materials?
How mirrors work
A mirror works by reflecting light. When light hits the surface of a mirror, it bounces off rather than passing through. To understand why silver is used for mirrors, it helps to know a bit about the physics of reflection.
When light hits an object, it can be transmitted, absorbed, or reflected. A transparent material like glass transmits much of the light – it passes through. An opaque object like a paved road absorbs most of the light, converting it to heat. A reflective surface like a mirror reflects the majority of the light.
Reflection occurs when light bounces off the surface of a material. How well a surface reflects light is determined by its reflectance. Reflectance measures what percentage of light is reflected. A perfectly reflective surface has a reflectance of 100% – it reflects all the light that hits it and absorbs none. Silver has one of the highest reflectances of any metal, reflecting 95-99% of light. This makes it an ideal material for mirrors.
Properties of silver
Silver has a number of properties that make it well-suited for use in mirrors:
- High reflectance – As mentioned above, silver has a reflectance of 95-99%. This means it reflects almost all light that strikes it, enabling clear reflections.
- Reflects all visible light wavelengths – Silver has a high, uniform reflectance across the visible light spectrum. It reflects red, blue, and green light equally, so mirrors show colors accurately.
- Resistance to tarnishing – Silver resists tarnishing better than other metals like copper or steel. Tarnishing would reduce reflectance over time.
- Ductility – Silver is very malleable and ductile. It can be rolled into thin sheets less than 0.1 mm thick without cracking.
- Adhesion – Silver adheres well to glass. A thin layer can be deposited that sticks securely.
In addition to these intrinsic properties, silver coatings offer practical advantages. Silver can be applied through a process called silvering. Silvering involves depositing a thin layer of silver onto a substrate through chemical reactions, sputtering, or electroplating. The process for silvering glass is well-developed.
Why not other metals?
While silver has the highest reflectance in the visible spectrum, why not use other reflective metals for mirrors? Why not gold, aluminum, chromium, nickel, or rhodium?
There are a few reasons silver is preferred over other metal coatings:
- Reflectance – Silver has the highest reflectance in the visible range. Other metals also reflect well, but not as much as silver.
- Cost – Silver is more affordable than other high-reflectance metals like rhodium. Gold or platinum would be impractically expensive.
- Tarnishing – Many metals like copper and brass tarnish over time. The sulfides and oxides that cause tarnishing would reduce reflectance.
- Manufacturing – Silvering is a mature process that can be performed economically on an industrial scale on glass. Other metals may be more challenging.
Here is a comparison of the reflectance of various metals:
As the table shows, silver consistently has the highest reflectance across the visible spectrum. It reflects the most light, so it makes the best mirrors.
Aluminum as an alternative
The primary alternative to silver for mirrors is aluminum. Aluminum has a slightly lower reflectance at 80-95%, but it has some advantages over silver:
- Lower cost – Aluminum is cheaper than silver, bringing down the cost of mirrors.
- Greater durability – Aluminum coatings are usually thicker and more durable than silver.
- Easier application – Aluminum can be applied through sputtering or evaporation deposition, simpler processes than silvering.
On the downside, aluminum reflects slightly less light than silver. It also oxidizes over time, reducing reflectance. Aluminum oxide can be cleared with a re-coating, however.
Most household mirrors today use aluminum coatings. The lower cost and greater durability makes aluminum a pragmatic choice. Silver is reserved for applications where the highest possible reflectance is needed, like in telescopes, lab equipment, and some high-end mirrors.
Other reflective coatings
While silver and aluminum dominate, other materials can also be used as mirror coatings. These include:
- Rhodium – Rhodium is a precious metal with a reflectance up to 85%. It is extremely rare and costly, so it is only used in specialty applications.
- Gold – Gold coatings have a reflectance of 70-85%. Gold mirrors were often used in ancient times.
- Platinum – Platinum has a reflectance between gold and rhodium, but its cost is prohibitively high.
- Chromium – Chromium has a reflectance around 65%. Chromium film can be applied through sputtering.
- Nickel – Nickel also has a reflectance near 65%. Its lower cost makes it an economical choice.
- Tin – Tin has a 55% reflectance. Tin mirrors were used in ancient times.
- Copper – Copper has a similar reflectance to tin but tarnishes quickly. Copper oxide is black in color.
None of these alternative coatings are competitive with silver or aluminum for most mirror applications. But they offer options for specialized products.
Silver makes the best mirrors due to its extremely high and uniform reflectance across the visible spectrum. It also resists tarnishing better than other metals. While expensive historically, the silvering process is now cost-effective for the production of high-quality mirrors.
Aluminum is the other go-to coating for mirrors. It has a slightly lower reflectance than silver, but offers great durability at a lower cost. Most mass-produced mirrors opt for aluminum coatings for this reason.
Other metals like gold, rhodium, chromium and nickel have their niche uses, but none reflect light as well as silver. Thanks to its unique optical properties, silver has cemented its status as the ideal material for mirror coatings.
So in summary:
– Mirrors use silver or aluminum coatings due to their high reflectance.
– Silver has the highest reflectance at 95-99% across visible wavelengths of light.
– Silver resists tarnishing and can be applied in very thin, uniform coatings on glass.
– Aluminum has a slightly lower reflectance but is cheaper and more durable than silver.
– Other metals like gold and rhodium work as mirror coatings but are impractical due to cost.
– The high and uniform reflectance of silver makes it uniquely well-suited for mirror coatings.