A mirror is an object engineered to reflect light in a way that preserves visual information, creating a recognizable image of the scene in front of it. Understanding how a mirror functions requires looking at how light interacts with materials and the specific components used in its construction.
Specular Versus Diffuse Reflection
The ability to form a clear image rests on specular reflection. When parallel light rays strike a perfectly smooth surface, the individual rays bounce off at the same angle they came in, remaining parallel. This uniform redirection allows the eye to perceive a crisp, coherent image, as if the light source were positioned behind the mirror. This process differs fundamentally from diffuse reflection, which is how you see objects like a wall or paper. When light hits a microscopically rough surface, the incoming rays scatter in many different directions, and this collective scattering prevents the formation of an image.
The Essential Material Components
A modern mirror is a composite structure, relying on a highly reflective metal and a durable, smooth support. The reflective layer is a thin coating of metal, most commonly silver or aluminum, applied to the back of a glass sheet. These metals are chosen because their atomic structure contains free electrons that are efficient at absorbing and immediately re-emitting light energy. Silver has the highest reflectivity across the visible light spectrum, reflecting up to 95% of incoming light. Aluminum is a less reflective alternative, typically reflecting around 90% of visible light, but it is harder and more resistant to tarnishing. The glass, called the substrate, does not perform the reflection; its purpose is to provide a rigid, smooth, and transparent surface to hold the delicate metal layer.
Engineering the Perfect Flat Surface
The quality of a mirror depends entirely on achieving an extreme degree of flatness on the glass substrate, known as optical flatness. Even microscopic imperfections can cause incoming light rays to scatter, degrading the sharp reflection. Manufacturers measure this flatness in fractions of a wavelength of light, with precision mirrors often achieving flatness measured in millionths of an inch. Because the metal reflective coating is placed on the back of the glass substrate, it must be protected from damage and corrosion. A multi-layered backing is applied over the metallic film to ensure longevity, preventing the metal from reacting with moisture and oxygen, which would otherwise cause the reflective layer to tarnish and develop black spots.