Popular answers to this question included “silver,” “white,” “whatever color it’s reflecting,” and “no color at all.” But most mirrors are actually very faintly green. Yes, green.
I love this question, because it reveals how a little bit of knowledge can, paradoxically, muddy our understanding of the world. What do I mean by that?
Consider that many people are at least somewhat familiar with how humans perceive color: The electromagnetic spectrum comprises a range of wavelengths that are visible and invisible to the human eye. The visible portion of the spectrum spans wavelengths from around 400 nm to 700 nm, with colors like violet, indigo, blue, green, yellow, orange, and red mapping to that spectrum in order of increasing wavelength:
When light comes into contact with an object, the object absorbs specific wavelengths from the visible spectrum. Any visible-spectrum wavelengths not absorbed are reflected, and reflected wavelengths that find their way to your eyes are perceived as any number of colors. Objects that absorb all visible wavelengths are perceived as black. Those that reflect all visible wavelengths are perceived as white.
Those are the basics. Now think back to our question: What color is a mirror? Mirrors, like things we perceive as white, reflect all visible wavelengths. But then why aren’t mirrors white—and why can’t you see your reflection in a sheet of printer paper? This is what I mean about knowledge begetting uncertainty: Most of us know just enough about color perception to arrive at difficult questions like these. And what a fantastic place to find yourself! It’s easy to think of knowledge as an antidote to confusion, but, more often than not, it’s also a stepping stone to other uncertainties—and further discovery.
In this case, the discovery is that there are different ways to reflect light. A white sheet of paper exhibits something called “diffuse reflection,” because, as the phrase suggests, the object reflects light diffusely, i.e. scattershot, in many directions. Mirrors, by comparison, exhibit “specular reflection.” The wavelengths that leave the mirror’s surface are organized according to the angle and configuration by which they arrived. I like how Phil Plait compares the phenomena, and the way he characterizes specular reflection as a sort of reconstitution process:
A white shirt just reflects light back everywhere in all directions. Even if red and blue light hit [a] shirt coming from the same direction, they may get scattered in different directions. A mirror, on the other hand, reflects the blue and red light in the same direction, and so the mirror actually builds an image of the source of the light.
Armed with this understanding, Plait concludes that mirrors are “a smart kind of white.” I really like this definition. But as VSauce’s Michael Stevens points out, it’s incomplete. It turns out the majority of mirrors are actually green.
Most mirrors that you encounter on a day-to-day basis, like the one over your bathroom sink, are composed of a soda–lime silica glass substrate and a silver backing. In their 2004 paper “Virtual tunnels and green glass: The colors of common mirrors,” researchers Raymond L. Lee, Jr. and Javier Hernández-Andrés refer to this substrate-backing combination as “the optical core of most common mirrors.”
This combination of materials is what gives mirrors a greenish hue. Though that hue is rarely perceptible, one place you can see it is in the infinitely repeating reflections of a mirror tunnel. Mirror tunnels emerge whenever mirrors face eachother, as in the photo at the top of this post. The Science Museum in Granada, Spain is home to a specially designed mirror tunnel that visitors can peer into through two small holes in the back of one of the mirrors:
Lee and Hernández-Andrés visited the museum and measured images generated by the multiple reflections inside the Science Museum’s mirror tunnel. They found that the mirrors best reflected light at wavelengths between 495 and 570 nanometers, which the human eye perceives as green. They found the same thing when they recorded measurements from common mirrors in their own laboratories and homes:
The x-axis on this graph denotes “spectral reflectance,” or how effective the surface is at reflecting electromagnetic waves. No surface reflects 100% of the light that hits it, so bouncing light back and forth and back and forth across two mirrors results in a gradual decrease in reflectance. You’ll notice the museum mirrors have the lowest spectral reflectance of all, because, as Lee and Hernández-Andrés note: “In the mirror tunnel, of course, light undergoes not one but many reflections between the two mirrors before reaching our eyes.” After fifty reflections, a white object’s reflected luminance was reduced by a factor of 5,780, and the dominant wavelength became 552 nanometers—which we perceive as a yellowish green.
Contact the author at rtgonzalez@io9.com. Top Photo: fereste | CC BY-NC-ND 2.0. Visible spectrum render by Spigget | CC BY-SA 3.0. Diffuse vs Specular reflection figure by GianniG46 | CC BY-SA 3.0. Photo of the Science Museum in Granada, Spain by Lee and Hernández-Andrés.