What Colour Is a Mirror?

A mirror is very faintly green. A perfect mirror would have no colour of its own, because it would reflect every wavelength equally and simply show you whatever is in front of it. Real mirrors are not perfect. The glass they are made from, and the metal coating behind it, reflect the green part of the spectrum slightly more efficiently than the red or blue parts, typically by a few percent. You cannot see that in a single reflection. Point two mirrors at each other and look down the tunnel of repeated reflections, and each bounce multiplies the bias until the far end is unmistakably green.

• A mirror is very faintly green because glass and its metal backing reflect green a few percent better than red or blue.
• One reflection hides the tint; a tunnel of ten reflections between two mirrors reveals it.
• A mirror is not white: white scatters light, a mirror reflects it in order and shows an image.
• In a dark room a mirror is black, because it has no light of its own.

That is the answer, and the mirror tunnel is the proof. Here is what is going on.

Because nothing reflects perfectly. An ordinary mirror is a sheet of glass with a thin layer of aluminium or silver on the back. Light passes through the glass, bounces off the metal, and passes back out through the glass. Every one of those steps loses a little light, and the losses are not identical for every wavelength. Glass absorbs a touch more at the red and blue ends than in the middle; aluminium reflects green a little better than either extreme. The result is a reflection that returns roughly ninety-something percent of the green light and a few percent less of the red and blue. The eye reads that as a very faint green tint, far too subtle to notice in one bounce.

By compounding. Put two mirrors face to face and look in at an angle and you see a corridor of reflections stretching away. Each image is the previous one reflected again, so the light in the tenth reflection has bounced ten times. If each bounce keeps ninety-five percent of the green and ninety percent of the red, then after ten bounces the green is at about sixty percent of its starting brightness and the red at about thirty-five. The tunnel drifts green, then a darker green, then a murky green-black at the vanishing point. That colour was always in the mirror. Repetition simply turned the volume up.

People often say a mirror is white because it reflects all colours, but the two are different kinds of reflection. A white wall scatters light in every direction, so it looks the same from any angle and shows no image. A mirror reflects light in an orderly way, each ray leaving at exactly the angle it arrived, which is why it shows an image and why its colour depends on what is in front of it. A mirror facing a red wall looks red. A white wall facing a red wall still looks white-ish, only tinted. So a mirror is not white. It is, as far as colour goes, almost nothing, with a faint green lean.

Black, or as near to black as the room is. A mirror has no light of its own. Everything you see in it is borrowed. Turn off the lights and the mirror shows you the darkness in front of it, exactly as faithfully as it showed you the room. This is the cleanest way to understand what a mirror's colour means: it is the colour of the light it hands back, minus a little at the red and blue ends.

Because it separates two ideas that usually get muddled: the colour of an object and the colour of the light leaving it. Most objects have a fixed colour because they absorb some wavelengths and scatter the rest; a lemon is yellow in any white light. A mirror has almost no fixed colour because it absorbs almost nothing and scatters nothing. What you see is the scene. The faint green is the only part that belongs to the mirror itself, and you need ten reflections to find it. Once you have seen that, the question "what colour is X" starts to sound different. The honest answer is often "what colour is the light, and what did X take away".

A coloured filter. A mirror keeps almost everything and shows it back. A filter keeps a specific slice of the spectrum and lets it through, throwing the rest away. A CMY Cube is a set of three filters on one object: the cyan face removes red, the magenta face removes green, the yellow face removes blue, and where they overlap they remove two at once. Hold one to a window and you are watching the exact process a mirror barely does, done deliberately and in bulk. Set it in front of a mirror and you can watch light go through the filter, off the glass, and back through the filter, losing colour on every pass, which is a small home version of the green tunnel.

A mirror is very faintly green because glass and its metal backing reflect green a few percent better than red or blue. One reflection hides it; a tunnel of ten reveals it. A mirror is not white, because it reflects in order rather than scattering, and it is black in the dark because it has no light of its own. Its true colour is almost nothing, which is what makes it a mirror.

Hold the opposite of a mirror, and see light through a different lens.