How silver turns light back
Silver’s atoms contain electrons that are relatively free to move through the metal. When incoming light reaches the coating, its rapidly changing electric field nudges those electrons back and forth. Their motion produces a new electromagnetic wave that travels away from the surface. The combined effect is that much of the light is sent back rather than absorbed.
Because visible light contains many wavelengths, a useful mirror must reflect a broad range of them. Silver does this especially well, which is why it can produce a bright reflection without needing a thick slab of metal.
Why the surface must be smooth
A mirror does more than return light: it must return neighboring rays in an orderly way. If the reflective layer has bumps large enough to scatter light in different directions, the reflection looks blurred or dull. A smooth silver film preserves the pattern of light coming from an object, so the eye can reconstruct its shape and position.
In a common glass mirror, light passes through the glass, reflects from the silver on the back, and passes through the glass again. The glass provides a flat support and shields the delicate metal from scratches and air. Without that protection, silver can tarnish as it reacts with substances in its surroundings, reducing the brightness of the reflection.
The familiar mirror is therefore a carefully paired surface: glass keeps the reflective film smooth and protected, while silver’s mobile electrons send the image-forming light back toward the viewer.
