How Do Mirrors Work?

We’re so accustomed to using mirrors, whether we’re checking our hair before heading out the door or catching our reflection in a store window, that we usually take them for granted. It’s not until you stop and really think about mirrors that they can start to feel like magic. 

For example, try taking a sheet of paper (or hand towel) and use your palm to press it against your bathroom mirror. At first glance, you’d think the paper would be completely obscuring your hand from the mirror, as it completely covers the space between your fingers and the glass. And yet, if you look from an angle, you can see the reflection of your hand as plain as day.

How can a mirror reflect something without a direct line of sight? Well, it’s not magic. It’s also not the same way your smartphone camera looks back at you when you’re taking a selfie.

The answer is all about how light reacts to surfaces, and how your brain reacts to light.

Law-Abiding Light

Everything you see is visible because light reflects off it and into your eyes, allowing your brain to process the information. (You can’t see objects in a dark room because there’s no light reflecting off them, but they’re still there.) When sunlight or a lamp illuminates your face, some of that light bounces back in every direction. When you stand in front of a mirror, some of those bouncing — or reflecting — rays of light strike its surface. That’s when the “magic” happens.

A mirror works in part because it follows the law of reflection: Light always bounces off at the same angle at which it arrives. If a ray of light hits the mirror at a 30-degree angle, it reflects away at a 30-degree angle. Every single ray follows this rule, creating a perfectly organized reflection — if the light coming off just your ear suddenly shifted 45 degrees, for instance, your reflection would look like a Picasso.

That predictability is only as exact as the surface light is bouncing from, though. (That’s why fun house mirrors don’t perfectly reflect you.) Mirrors need to be astonishingly smooth. Most household mirrors are made from a sheet of glass coated on the back with a thin layer of aluminum or silver. The glass protects the delicate metal coating, while the metal reflects most of the light. 

Metals such as aluminum and silver are especially good reflectors, accurately sending most of the incoming light back instead of absorbing it. That’s why you can see through a window but not a mirror — the glass isn’t what’s doing most of the reflecting.

On a microscopic scale, the reflective surface of metal in a mirror is so smooth that its tiny imperfections are smaller than the wavelength of visible light. As a result, the light rays bounce off together in an orderly pattern instead of scattering in different directions.

This also explains the “obscured hand” illusion. Even though a sheet of paper is blocking the mirror directly in front of your hand, light is still bouncing off your hand at more extreme angles, and uncovered parts of the mirror are receiving that light. From there, they reflect back toward your eyes, which is why you see a reflection of your hand despite the paper “hiding” it from the mirror.

Not only do you still see your hand, but it appears to be coming from the other side of the towel, beyond the plane of the mirror. So why does your reflection appear to come from behind the mirror if reflected light originates from its front surface?

Fast Reflects

Your brain assumes that light travels in straight lines. When light rays bounce off a mirror and into your eyes, your brain mentally traces them backward, making it seem as though they’re coming from a point behind the glass. This creates what’s called a virtual image — an image that appears to exist in space but isn’t actually there. No light is coming from behind the mirror at all; it’s simply an illusion created by the paths the reflected rays take.

Not all mirrors produce the same kind of reflection. A flat, or plane, mirror creates an upright image that’s the same size as you are. Curved mirrors change the paths of light in different ways. A concave mirror (which curves slightly inward) collects more light, so it magnifies objects — that’s why it’s used as a makeup mirror. A convex mirror (curving slightly outward), shrinks images but creates more room for a “wide-angle” view, making it useful for side mirrors on your car.

All these rays of light are still following straight lines, though. Mirrors don’t create an image from scratch or store a picture of you inside the glass. It’s simply and obediently following the laws of physics, directing light with extraordinary precision — reflecting every movement you make at the speed of, well, light.