A modern mirror is a composite structure, not simply a piece of glass. The reflection is created by a metallic layer meticulously coated onto the glass surface, which captures and bounces back light. While glass mirrors are relatively new, the concept is ancient, with early reflections achieved using still water or highly polished discs of metal like bronze or silver. The glass itself is transparent and plays a supporting role.
The Essential Function of the Glass Base
The primary purpose of the glass in a modern mirror is to provide a perfectly flat, rigid, and smooth substrate. Glass is an ideal material for this role because its hardness allows it to be polished to an exceptional degree of flatness without warping. This smoothness is important, as any microscopic irregularity would cause light to scatter, resulting in a distorted image.
The rigidity of the glass maintains the precise geometry of the reflective coating, preventing bending or flexing that would introduce distortion. The glass acts purely as a stable foundation. High-quality float glass is commonly used because it has uniform thickness and lacks internal imperfections that could interfere with the reflection process.
What Makes the Mirror Reflective
Reflection is solely due to an extremely thin layer of metal applied to the glass substrate. Historically, mercury was used, but modern mirrors utilize silver or aluminum due to their superior reflective properties and safety. Silver, for example, offers over 95% reflectivity for visible light.
This metallic layer is applied through processes like “silvering,” a chemical deposition method for silver, or vacuum deposition (sputtering) for aluminum. Vacuum deposition involves vaporizing the metal in a vacuum chamber, allowing it to condense as a uniform, microscopic film onto the glass surface. This film is measured in nanometers and is entirely responsible for bouncing light rays back to the viewer.
Because silver and aluminum are prone to oxidation and damage, the metallic film requires protection. A thin layer of copper is often applied over the metal for adhesion and stability. Finally, a protective layer of paint or lacquer is applied over the copper to seal the reflective components from the environment, ensuring the mirror’s longevity.
Two Main Types of Mirror Construction
The placement of the reflective coating defines the two main types of mirror construction: second-surface and first-surface.
Second-Surface Mirrors
The common household mirror is a “second-surface mirror,” meaning the metal coating is applied to the back of the glass. The glass protects the fragile metal from scratches, dust, and corrosion, making it suitable for everyday use. The trade-off is that light must pass through the glass twice, which can cause a slight loss of light and a faint secondary reflection, known as “ghosting.” Second-surface mirrors typically reflect about 80–85% of incoming light.
First-Surface Mirrors
A “first-surface mirror” has the reflective coating applied directly to the front surface, facing the light source. This arrangement is used in specialized equipment like telescopes where maximum light reflection and image clarity are needed. Since the light does not pass through the glass, there is no ghosting or light loss, often yielding 94% to 99% reflectivity. However, because the delicate metallic film is exposed, first-surface mirrors are much more vulnerable to damage and require careful handling.
Reflective Surfaces That Are Not Glass
Since glass is merely a substrate, many other materials can be used as a base for reflective surfaces. Long before modern glass mirrors, highly polished metals, such as bronze or copper discs, provided reflections. These polished metal surfaces still function as mirrors today, demonstrating that the substrate material is secondary to the reflective coating or polish.
Modern alternatives include mirrors made with a flexible polymer or plastic substrate, such as Mylar or acrylic sheets. Acrylic mirrors are significantly lighter and more shatter-resistant than glass, making them suitable for safety applications, though they may introduce slight image distortions. Specialized “glassless mirrors” consist of a metallized plastic film stretched over a lightweight frame, used where weight is a concern, such as stage or ceiling installations.