The common mirror hanging on a wall is known as a second-surface mirror, meaning the reflective material is located behind the transparent glass. This construction protects the delicate reflective layer from damage and the environment. The back of a mirror is a sophisticated combination of materials, not merely paint, that work together to produce and preserve the image you see. Understanding these specific layers is key to appreciating how this everyday object functions and maintains its clarity over time.
The Essential Reflective Layer
The foundation of any mirror’s ability to reflect light is a coating of metal applied directly to the back of the glass substrate. The two primary metals used are silver and aluminum, chosen for their high reflectivity across the visible light spectrum. Silver offers superior performance, reflecting approximately 95% of incident light and providing a neutral color tone. Silver is applied using a chemical process called silvering, which deposits a uniform metallic layer from a silver nitrate solution. Aluminum is more common for modern, mass-produced mirrors because it is more cost-effective and durable. It is typically applied using vacuum deposition, a process that vaporizes the metal inside a vacuum chamber to create a fine coating.
The Role of Protective Backing Materials
Immediately following the metal application, protective coatings are applied because reflective metals, especially silver, are highly susceptible to oxidation. Oxidation creates the dark spots and discoloration seen in damaged mirrors when the metal reacts with oxygen and moisture.
A chemical passivation layer or copper is often applied directly over the reflective metal, acting as a barrier to chemically shield the silver or aluminum from corrosive elements. In modern, copper-free construction, specially formulated chemical layers achieve the same anti-corrosion effect.
The final, and most visible, layer is a durable, protective paint or lacquer. This paint is applied over the chemical or copper layer to provide a tough physical shield against abrasion, cleaning chemicals, and moisture penetration, sealing the entire assembly.
How Different Mirror Types Are Constructed
The standard second-surface mirror design is a practical compromise between reflection quality and durability. The glass acts as a permanent, hard shield, allowing the mirror to withstand regular cleaning and environmental exposure. However, light must pass through the glass twice, which can cause a slight distortion, known as “ghosting,” and reduces brightness, with reflectivity typically ranging from 80% to 85%.
In contrast, a first-surface mirror has the metallic coating applied to the front surface of the glass, facing the viewer. Since light reflects directly off the metal without passing through the glass, these mirrors eliminate ghosting and often exceed 94% reflectivity. This design is employed in specialized applications, such as telescopes and laser systems, where image accuracy and light efficiency are paramount.
Because the reflective layer of a first-surface mirror is exposed, it is far more fragile and susceptible to scratching and tarnish. To combat this, a thin, transparent dielectric overcoat is sometimes applied directly over the metal for protection. This structural difference means everyday mirrors prioritize longevity, while specialized mirrors prioritize maximum clarity.