Fascinating question, that. What color is a mirror? Hard to say, really. I mean, are we talking about a regular mirror you might pick up at IKEA, in some misguided attempt to faux enlarge your little studio flat? Are we talking about an antique mirror, one made of some exotic material that reflects back in shades of copper or bronze? Or are we talking about a trendy little number, tinted in one or more psychedelic colors to blow the minds of the crowd at your local dance club?
Well, to answer such a question, it usually helps to decide in the first place what the question is actually asking. In this case, we need to have a workable definition of the words color and mirror. If we don’t, we’ll never know what “color” the “mirror” is, will we?
The Colors, the Colors . . .
As a nod to you britnoders, I will acknowledge that the word “colour” is used in certain civilized portions of this planet. However, I will ask everyone to accept my Americanized version of the word for the duration of this writeup.
What is color? The standard, Wikipedia definition tells us that it is a “property of light that is determined by its wavelength.” Not very helpful? Let’s try this.
It is the property or characteristic assigned by the human brain to different wavelengths of light, or electromagnetic radiation, within the visible spectrum. So if light coming into our brain through our eyes has a wavelength of roughly 700 nanometers (nm), our brain perceives that light as red. If the light has a wavelength of 530 nm, our brain perceives it as green. And if the light has a wavelength of 420 nm, our brain perceives it as violet.
What is the “visible spectrum?” It’s the portion of the electromagnetic spectrum visible to the human eye. For humans, that starts somewhere between 1,000 nm and 700 nm, which is the line between infra-red and red, and ends somewhere between 420 nm and 300 nm, which is the dividing line between violet and ultra-violet. In between those extremes you’ve got ROYGBIV. For the non-physics majors out there, that’s red, orange, yellow, green, blue, indigo, and violet. All the colors of the rainbow.
So when your brain tells you you’re seeing a red ball in the street, what you’re really doing is looking at a spherical object made of a material that absorbs every wavelength of light except red. When your brain tells you you’re seeing blue curtains hanging from the wall, what you’re really doing is looking at draped fabric that absorbs every wavelength of light except blue. And when your brain tells you you’re looking at an eggplant duvet cover, what you’re really doing is discovering that you’re a gay interior designer, but you just haven’t admitted it to yourself yet.
White Light, Bright Light
We learned about ROYGBIV, but what color is white? Or black, for that matter? Well, what we call white light is actually made up of all the different colors of light along the visible spectrum, because when all the wavelengths of the visible light spectrum strike your eye at the same time, your brain perceives them as white. Don’t ask me why, I’ve never figured that one out. But technically speaking, white is not a color at all, simply the combination of all the colors of the visible light spectrum.
If all the wavelengths of the visible light spectrum give the appearance of white, then black would necessarily be the opposite –- the absence of wavelengths of light lead to the appearance of black. As with white, black is not actually a color, but is simply the absence of the wavelengths of the visible light spectrum.
So when you are in a room with no lights and everything around you appears black, it means that there are no wavelengths of visible light striking your eye as you look at your surroundings. Or when you look at a black object, say a briefcase, what that means is that the object is absorbing all of the electromagnetic radiation that hits it. That’s why you don’t wear black in bright sunlight. Conversely, anything that appears white to you absorbs nothing, and is simply reflecting all of the colors of the visible spectrum back at your eyes.
Oh, one more thing. Most objects, white, red, violet, or anything in between, reflect light back in all directions. The light hits the object –- whatever it may be –- and is scattered randomly back to the eyes of any observers. This means that, for most objects, a coherent, discernable pattern is not going to be visible in the reflected light.
Mirror, Mirror . . .
The same cannot be said for a mirror, which reflects light back in a consistent, uniform pattern sufficient to create a reflection of any light source in its view. But what is a mirror? Well, the Wikipedia definition is “a device whose surface has good specular reflection.” Once again, not very helpful.
A better definition of a mirror might be an object that has a sufficiently uniform and reflective surface to reflect all light hitting it in a specific, coherent manner. For a plane mirror -– such as the flat mirror you probably looked at while brushing your teeth this morning -– this “coherent reflection” means that the angle of the incident light (light hitting the mirror) will equal the angle of the light reflected from the mirror.
As an imperfect example, think of the light hitting the mirror as a pool ball hitting the bumper sides of a pool table. If the ball (light) comes in at a 60º angle to the bumper (mirror), it will leave at an equal (60º) and opposite angle from the bumper. I say imperfect because light is neither a wave nor a particle, but exhibits characteristics of both. A pool ball, on the other hand, is definitely limited to the characteristics of a particle. Nonetheless, the mental picture of a pool ball bouncing around a table gives a fair approximation to how light works when shone against a mirror.
So a mirror is something that reflects light in a uniform, predicable manner. Does a mirror have a color? Well, some mirrors certainly do. There are tinted and shaded mirrors that might reflect the shape and features of an object, while at the same time making that object appear red, or green, or whatever shade of tinting is used. In terms of electromagnetic radiation, this simply means is that the tinting is absorbing light along the entire spectrum except light that is the color of the tint itself. So a green mirror absorbs everything but green light, and so forth.
But what about a plain old mirror, one with no discernable color, shading or tinting at all? Does that kind of mirror have a “color?” Yes, and it’s not just the color of whatever object you happen to put in front of the mirror.
No, regular mirrors reflect the entire range of visible light. With no tinting to absorb extraneous wavelengths, mirrors will take any wavelength of light and bounce it back off their surface. Just as any “white” object would do. The difference, though, is this: mirrors bounce light back in a very specific pattern.
So if you put a piece of white paper and a mirror next to each other on a table, they certainly won’t look the same. The piece of paper will bounce electromagnetic radiation back in a scattered, haphazard fashion, one that our brain perceives as white. The mirror will bounce the exact same range of electromagnetic radiation back off its surface, but will do so in an organized fashion that produces a reflection of the light source, rather than white light. But the simple fact is that neither the mirror nor the paper is absorbing anything along the visible spectrum of electromagnetic radiation. So in terms of how we normally define color, they’re both “white.”