Most of us use mirrors every day without stopping to reflect on how they actually work. Why is it that mirrors reflect images of their surroundings when other objects don’t? Why can we see ourselves in mirrors, and what’s actually happening when we look into a looking glass?
Considering the near-magical function mirrors perform, their construction is surprisingly simple. Most household mirrors are made of glass with a thin layer of metal backing (usually aluminum), and several layers of paint. It turns out glass isn’t the most important component of a looking glass. Instead, the glass surface of a mirror performs a predominantly protective function, preserving the extremely thin, extremely smooth layer of metal behind it. Light passes through the glass part of the mirror and is reflected by the metal. The layer of paint at the back of the mirror serves a similar protective function, keeping the metal in place.
But why are mirrors uniquely reflective? When light hits a mirror, it reflects every color in the visible spectrum. Most objects absorb some colors and reflect others, giving rise to our perception of the color properties of things. For example, when light hits a banana, it absorbs every color except for yellow, which it reflects, making the banana appear yellow. You might also remember from school that, much like mirrors, white objects (like a piece of printer paper or a white wall) reflect all the colors of the visible spectrum.
The reason mirrors are reflective and other flat white surfaces aren’t is because they’re smooth on a microscopic level. While surfaces like walls or paper may look smooth to the naked eye, if you zoom in close enough, they’re actually quite bumpy. When rays of light hit rough surfaces, they bounce the light back in all directions. This is called diffuse reflection. Metal and glass, meanwhile, are very smooth, and reflect light back more directly. This is called specular reflection. If that’s hard to visualize, imagine throwing a bunch of tennis balls at a wall. If all of the balls are thrown at a straight angle, you’d expect them all to bounce back at the same angle, no matter where they hit the wall. Now imagine throwing the tennis balls at an uneven surface like a craggy rock face—depending on where they hit, the balls will bounce back at different angles. Their trajectories will be different because they’re hitting an uneven surface.
The same principle is at work when light hits other smooth surfaces, like a calm body of dark water. If you look into a lake on a windless day, you’ll be able to see your reflection because the smooth surface of the water is producing a specular, rather than diffuse, reflection. But if a strong gust of wind comes and ripples the water, your reflection will become distorted, or more diffuse.