What color is a mirror? It sounds like one of those deep, paradoxical questions a Buddhist monk might ponder on top of a mountain, but the answer is actually surprisingly straightforward: it's a faint shade of light green.
At least that’s the case with most mirrors you probably encounter on a regular basis. The majority of household mirrors are constructed using a soda-lime silica glass substrate and a silver backing. This combination is what gives mirrors their greenish hue, though you wouldn’t know it just by staring at your own reflection.
The shade becomes noticeable when two mirrors are placed in front of each other, creating the seemingly infinite number of reflections known as a mirror tunnel. In their 2004 paper, researchers Raymond L. Lee, Jr. and Javier Hernández-Andrés talk about paying a visit to the Science Museum in Grenada, Spain to measure images generated by the mirror tunnel there. They discovered that the mirrors best reflected light at wavelengths between 495 and 570 nanometers, which is what the human eye perceives to be green.
As light bounces back and forth from one mirror to the next, the mirror’s reflective capabilities gradually weaken. If someone is looking at the reflection produced in a mirror tunnel, the light waves have already been reflected several times over before reaching their eyes, thus making the greenish color of the mirror’s material more prominent.
If you answered “white” to the question, “what color is a mirror?” that wouldn’t necessarily be wrong either. White is the color that reflects all the visible wavelengths that make up the color spectrum. The reason that you can’t see your reflection in a sheet of paper is because white objects scatter light in all different directions, while mirrors reflect light back in the same direction they came from. “Bad Astronomer” Phil Plait describes mirrors as a “smart kind of white.” Now that this mystery has been solved, it's time to shift focus to bigger questions like, “what’s the sound of one hand clapping?” and “why do cats purr?"