What Colour is a Mirror?
A question so seemingly simple, yet infuriatingly confusing when actually contemplated.
I think it’s safe to assume we’ve all looked in a mirror before—probably a couple of times just today. But when have you actually stopped to think about the colour (or anything at all, really) of the surface we know to show us what we can’t directly see?
The easy answer is a mirror is the colour of whatever appears on it! If you put a blue cloth in front of a mirror, the mirror will be blue! Unfortunately, that’s not very scientific, is it? Plus, that’s describing the reflected image, not the mirror which we’re interested in, though this might be the reason it’s so hard to imagine its true colour. Have you ever seen a mirror not show a reflection of another object’s light rays? I know I haven’t.
Nevertheless, there are some common misconceptions on what the colour we’re looking for might be. The typical mirror is made of soda-lime glass and a smooth, shiny silver backing, so it makes sense for its colour to be silver. If you search up images of mirrors in google, you’ll see tons of frames with a silver block in the middle. However, that’s not exactly true. Okay… then following a more scientific train of thought, a mirror is similar to the colour white in the sense that they both reflect back all the colours (wavelengths) of white light, so a mirror should be white, right? Not quite, but we’re getting closer. In fact, mirrors are often simplified to be thought of as a “smart white” colour. Really, nothing can be perfectly white because nothing is 100% efficient at reflecting all light headed towards it (energy loss).
So, where does that leave us? It’s not silver or white. Is it clear or colourless? No. Is it somehow black? No. Is it green? Yes.
Wait, what? Green?
As unbelievable as it sounds, yes, a mirror is slightly green. Let’s see how.
Before finding out how we got to this conclusion of green, we should understand how mirrors work in relation to light and colours.
The way a mirror reflects light is different from everyday objects like the yellow sticky notes sitting on your table. The sticky notes reflect light diffusely, or in various directions so that a clear image of the source object is not produced. On the other hand, a mirror exhibits specular reflection. This type of reflection directly correlates to the angle at which light rays hit the surface of the mirror as the incident ray angle will be practically equal to the reflected ray angle (so long as the mirror is smooth and flat) and this allows us to see a clear, reflected image of all objects whose light rays bounce off the mirror and into our eyes.
We know that colours are perceived based on which light wavelengths they reflect. A sticky note absorbs all wavelengths except for the specific hue of yellow that we attribute to it, which is then reflected so that our eyes can process it. A perfectly black object will absorb all wavelengths and a perfectly white object will reflect all wavelengths of the visible spectrum of light (again reinforcing the idea that a mirror is a “smart white”).
Taking a look at the spectrum of light reflected by a typical soda-lime glass mirror, we can see that it best reflects light within the 510-nanometer wavelength range. What colour does this represent? You guessed it. Green. Therefore, though it is not explicitly noticeable, a mirror is technically green.
Where does this innate “greenness” of common mirrors come from? The answer is actually quite simple—the glass. I mean, it’s called soda-lime glass. I should’ve put a spoiler warning on that earlier on. Jokes aside, “soda-lime” simply refers to the chemical composition of the glass and it actually gains this green tinge from tiny impurities of iron in the substance.
Now that you know the true hue of a mirror, you might actually want to see it for yourself. The easiest way to do this is by viewing an infinite mirror tunnel. This is an arrangement of multiple parallel or near-parallel mirrors which create a never-ending self-reflecting image that decreases in size as it seems to gets farther and farther away.
As the reflections compound, the image beings to appear more and more green as with each snowballing reflection some visual light is lost, out of which green is lost the least.
So, does this mean we don’t know the absolute precise colour of our outfit each morning? Even though it’s such a minuscule difference you really wouldn’t notice, that’s a scary thought if I’ve ever had one.
