You look in the bathroom mirror every morning and probably never think twice about the face staring back.
Glass itself reflects a small portion of the light that reaches it, which is why you can sometimes see reflections in a window. But most visible light passes through ordinary clear glass. A household mirror adds a highly reflective layer that sends much more of that light back toward you.
The metal is doing most of the work
A typical household mirror consists of a sheet of glass with a thin reflective coating on the back. That coating is commonly made from silver or aluminum, although mirror construction varies.
Metals are highly reflective because their mobile electrons respond to the electric field of incoming light. This interaction produces electromagnetic waves that travel back away from the metal, giving metals their strong reflective properties.
The glass serves several purposes. It provides a smooth, rigid surface for the reflective layer and protects it from damage on the viewing side. Because the coating sits behind the glass in an ordinary household mirror, light passes through the glass before reaching the reflective layer, then passes through it again on the way back to your eyes.
Why the image looks sharp
A clear reflection requires specular reflection, which occurs when light reflects from a sufficiently smooth surface in orderly directions.
Compare a mirror with a sheet of white paper. Both reflect light, but paper has a microscopically irregular structure that scatters light in many directions. A mirror's smooth reflective surface preserves the spatial information carried by the incoming light, allowing an image to form.
Scratches, warping, imperfections in the coating, or uneven glass can disturb that reflection. Depending on their size and severity, they may produce visible scratches, distortions, haziness, or other defects.
Angle in equals angle out
The basic rule is called the law of reflection. The angle at which a ray of light reaches a surface equals the angle at which it reflects, with both angles measured relative to a line perpendicular to the surface.
On a flat mirror, this geometry produces an upright virtual image that appears to be the same distance behind the mirror as the object is in front of it.
Curved mirrors obey the same law. The difference is that the surface's orientation changes from point to point, so reflected rays can converge or diverge. Depending on the mirror's shape and the object's position, the resulting image can appear larger, smaller, upright, inverted, or distorted.
Why left and right seem swapped
People often say that a mirror flips left and right, but that description is misleading.
A plane mirror reverses the direction perpendicular to its surface. Something extending toward the mirror appears to extend away from it in the reflected image. It does not selectively exchange your left and right sides.
The confusion becomes clearer when you imagine another person facing you. To stand in the same orientation as your reflection, you would normally imagine turning yourself around, and that rotation swaps which side appears left and which appears right from your viewpoint.
That is why writing looks reversed in a mirror even though the mirror has not performed a special left-to-right flip.
Not all mirrors are equal
The quality and appearance of a reflection depend on the reflective material, the glass, the protective layers, and the manufacturing process.
Silver is an excellent reflector across much of the visible spectrum, while aluminum is also highly reflective and is widely used in mirrors and optical applications. Modern mirrors can include additional coatings to protect the reflective layer from moisture, oxidation, scratches, and other damage.
Ordinary glass can also introduce a slight green tint, especially when light travels through a greater thickness of it. Low-iron glass reduces this effect when more neutral color reproduction is important.
No ordinary mirror reflects 100 percent of the light that reaches it. Some light is absorbed, transmitted, or lost at each reflection. That is why two mirrors facing each other produce a seemingly endless tunnel of images that progressively dim rather than remain equally bright.
Next time you catch your reflection, remember that the glass and its reflective coating work together. The coating reflects much of the light back, while the mirror's smoothness and shape determine where that light goes. What looks like a simple image in the bathroom is really a remarkably orderly journey of light from you, through the glass, to the reflective layer, and back to your eyes.
