How Mirrors Work


A mirror is any smooth surface that reflects back an image of an object. This happens when the light rays – called photons – coming from that object (for example, your face) hit the smooth surface of the mirror and bounce back at the same angle at which they hit the object. When the photons hit a rough surface they scatter and so can’t cause a reflection.

But not every smooth surface creates a reflection. That’s because some of these surfaces absorb the photons hitting them so they’re unable to bounce back. What’s more, when we do stand in front of a reflecting surface like a mirror, not all the photons that bounce off us will hit the mirror, because our bodies are rough objects themselves. But enough do bounce back that an image is formed, at exactly the same angle at which the photons hit the mirror. And keep in mind, the mirror image is actually backwards, as you can easily see if you stand in front of it holding a newspaper or magazine.

Mirrors are usually made of clear plate glass that has one side coated with a metal film to create the reflecting surface, called ‘silvering’. Where the surface and the glass meet is called the ‘mirror line.’ The most common type of mirror is the plane mirror, which has a flat surface. The rays of light that hit it are reflected back virtually unchanged, so the position of the image in the mirror is the same distance as the real object in front of it, and its size is the same size. You experience this every time you stand in front of the mirror in the bathroom – the closer you stand to the mirror, the closer your image appears in it.

Convex and concave mirrors – also known as ‘spherical mirrors’ – alter the look of the original object and the distance it appears to be in the mirror. A concave mirror (think of it as being ‘caved in’) has the mid-point of the reflecting surface farther away from the object in front of it than the edges. So the image that forms in a concave mirror depends on where it is in relation to the center of the mirror. It may be larger, smaller, or the same size. It may be upside down or right side up, and will be distorted from the real object.

Conversely, a convex mirror has the mid-point of the reflecting surface closer to the object than the edges, so the image that forms is consistently smaller than the real object in front of it, and standing upright. It will also be distorted. For example, drivers will stick a small convex mirror on their car or truck’s rear-view mirror to get a ‘bigger picture’ of what’s behind them than a regular, plane-type mirror shows.

Two-way mirrors are a special kind of mirror, usually used for observation and security purposes. They are sometimes also called one-way mirrors or see-through mirrors. You’ve probably seen them in action on tv shows or in movies where an interrogation is taking place. The perp is questioned in a room that has a mirror, while detectives observe him from behind the mirror in an adjoining room. So how come the criminal sees only his reflection but the cops see clear through the glass?

The answer is actually pretty simple. The two-way mirror in fact has only a very thin layer of reflective material applied to the glass. In fact, it’s so thin it’s referred to as being ‘half-silvered,’ as opposed to ‘silvered’ which is how you’d describe a regular mirror. So only half of the light that hits the mirror gets reflected, while the other half passes through it.

But with only half the reflecting power, you’d think the criminal would be able to see the detectives. The reason why he can’t is the way the two rooms are lit. If you watch the interrogation scene, you’ll notice the room with the criminal is brightly lit, so there is enough light to hit the mirror and reflect back his image. However, the room with the detectives is dark, so there’s not enough light to pass through that side of the glass back into the interrogation room.

Here’s another way to think of it: if the perp were to suddenly jump up and switch off the lights in the interrogation room, or if one of the cops accidentally turned on the lights in the observation room, the two-way mirror would act like a window, with everyone being able to see each other perfectly clearly.