How Do Mirrors Work?

My son loves mirrors. Not to the point that he spends hours in front of the mirror, mind, but he finds them interesting. He also thinks they work because of electricity, because both my car and my wife’s car are OnStar capable and so have a thick cable that connects the rear view mirror to the roof of the car. I haven’t been able to disabuse him of this notion, because he insists that I’m wrong when I tell him that electricity has nothing to do with reflection.

To tell the truth, I’ve been looking forward to this one myself. A lot. Because mirrors fascinate me, too. I’ve spent a lot of time in front of them, craning my head and discovering that I can see things in them that I wouldn’t have expected to be able to see reflected. You know, like how you can see into the living room a little by craning your head as you look into the bathroom mirror. They seem spooky at times, even though I know there must be a rational explanation.

Really, there is. It starts with the mechanics of how we see, and ends with physics.

How We See

Sight is a staggeringly complex concept that we generally take for granted. The National Eye Institute, part of the National Institutes of Health, provides a basic primer on how we see:

Light passes through the cornea, the clear, dome-shaped surface that covers the front of the eye. The cornea bends – or refracts – this incoming light. The iris, the colored part of the eye, regulates the size of the pupil, the opening that controls the amount of light that enters the eye. Behind the pupil is the lens, a clear part of the eye that further focuses light, or an image, onto the retina. The retina is a thin, delicate, photosensitive tissue that contains the special “photoreceptor” cells that convert light into electrical signals. These electrical signals are processed further, and then travel from the retina of the eye to the brain through the optic nerve, a bundle of about one million nerve fibers. We “see” with our brains; our eyes collect visual information and begin this complex process.

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That light has to come from somewhere, of course. While there can be many different sources of the light our eyes uses, those sources are ultimately one of two different categories: luminous objects, or illuminated objects. Luminous objects generate their own light, like a lightbulb or the sun. Illuminated objects are objects that reflect light, like the moon. Or mirrors. Or, really, anything at all that you can see.

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Pictured:  luminous and illuminated objects

What is reflection?

Dictionary.com provides the following definition of reflection:

Physics, Optics.

  • the return of light, heat, sound, etc., after striking a surface.
  • something so reflected, as heat or especially light.

It’s a little more complicated than that, but overly complicated. Reflection depends on something called the law of reflection, which states that “when light falls upon a plane surface it is so reflected that the angle of reflection is equal to the angle of incidence and that the incident ray, reflected ray, and normal ray all lie in the plane of incidence”.

Clear? If not, the University of Texas has you covered. Start with this image:

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There’s geometry there, and if you’re like me you haven’t done any geometry since high school. But really, the concept isn’t difficult. The incident ray is the ray (light, in this case) that strikes the reflecting surface. Assuming the surface is smooth, the reflected ray bounces off at the same angle as the incident ray.

And if the surface isn’t flat?

Well, in that case, the law still holds. Imagine zooming in on a rough surface – a rough-cut block of wood, for instance. At a small enough level, there are flat surfaces. Each flat surface becomes the plane for this law, and then the light is reflected accordingly. The rougher the surface, the more chaotic the reflected rays. The smoother the surface, the more uniform the reflected rays. This is the key to the two types of reflection:

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Specular reflection is simply reflection from a surface that makes the majority of the incident rays travelling in the same direction reflect from the surface in the same direction. Diffuese reflection, on the other hand, is when the reflected rays scatter in different directions. Now, clearly, no surface is perfectly reflective. But mirrors produce significantly more specular reflection than diffuse reflection.

So why do things look reversed in a mirror?

It turns out that this is nice and simple. HowStuffWorks: Science explains this pretty well, but I’ll take a stab at it myself. In essence, it has to do with the fact that the mirror is just reflecting back light that reflects off an object. The light doesn’t flip around, so you see the right side of a reflected object (your body, for instance) on the left side of the mirror.

Hmm… still not clear. Here’s what HowStuffWorks said to help clarify it further:

Take a piece of thin, translucent paper and write your name on it. Stand in front of a mirror and hold the paper up so that you can read the paper normally. Now look in the mirror. You are seeing the back of the translucent sheet in the mirror, and the word is not reversed — it looks completely normal. Now turn the paper over and look at it in the mirror. It is reversed, but so are the letters on the back of the translucent sheet. Note that you turned the paper over — you reversed it!

Explained that way, it makes perfect sense. To me, anyway.

But what about the witchcraft?

Have you ever noticed that you can see things in mirrors that look like they are way, way off to the side?  Things that look like they shouldn’t be able to be seen?  This always felt like witchcraft to me.  I mean, I’ve always felt that there had to be an explanation that wasn’t black magic, but I didn’t know what it was.  Well, it turns out to be all about lines of sight and the law of reflection. If you’re standing to one side of the mirror, you see the reflected rays that have a flatter angle of reflection. Those reflected rays are created by incident rays that have a flatter approach angle relative to the plane of the mirror. But then your line of sight “in” the mirror is a line that follows the reflected rays “through” the mirror, making it appear that the reflected ray originated from an object inside the mirror.

All in all, it’s no wonder my son is fascinated by mirrors. They just don’t do what you think they’ll do. Instead, they do what physics thinks they’ll do, and that is usually a completely different experience.

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