Mirror Physics

In order to understand mirrors, we first must understand light. The law of reflection says that when a ray of light hits a surface, it bounces in a certain way, like a tennis ball thrown against a wall. The incoming angle, called the angle of incidence, is always equal to the angle leaving the surface, or the angle of reflection. When light hits a surface at a low angle -- like on a lake at sunset -- it bounces off at the same low angle and hits your eyes full blast, rather than obliquely as when the sun sits overhead. This is why the sun's glare during the evening and morning is so much more intense than during the rest of the day.

Light itself is invisible until it bounces off something and hits our eyes. For instance, a beam of light traveling through space can't be seen from the side until it runs into something that scatters it, like a cloud of hydrogen or a satellite. This scattering is known as diffuse reflection and this is how our eyes interpret what happens when light hits an uneven surface. The law of

reflection still applies, but instead of hitting one smooth surface, light is hitting many microscopic surfaces. It's more like throwing a handful of marbles against a statue and then guessing the statue's shape based on how the marbles bounce.

Mirrors, however, don't scatter light in this way. With a smooth surface, light reflects without disturbing the incoming image. This is called specular reflection. That concept raises an interesting question: If mirrors preserve the images that hit them, why do they turn left into right and vice versa? Why not up and down? The truth is that a mirror doesn't really reverse left and right. What mirrors switch is front and back, like a printing press or a rubber stamp. Imagine writing something on a sheet of paper in dark pen and then holding it up to a mirror. It looks backward, but it also looks the same as if you held it up to a lamp and looked at it from behind. Your mirror image is a light-print of you, not a reflection of you from the mirror's perspective.

Mirrors can be more than just flat surfaces.

Types of Mirrors

One quick way to change the way a mirror works is to curve it. Curved mirrors come in two basic flavors: convex and concave. A convex mirror, which bulges outward, reflects at a wider angle near its edges than at its center, creating a slightly distorted image that's smaller than actual size. Convex mirrors have many uses. The smaller size of the images means that you can see more with these surfaces, hence their use in safety mirrors. (This is why your passenger side mirror says that objects are closer than they appear.) Some department stores have reportedly placed convex mirrors in their dressing rooms. Why? Slight bends at the top and bottom make you look taller and thinner.

Concave or converging mirrors curve inward like a spoon (the side that holds soup). This gives these mirrors the ability to create an image when their curvature bounces light to a specific area in front of them. This area is called the focal point. From far away, objects will seem upside down, but as you get closer and pass the focal point, the image flips and magnifies. Concave mirrors are used in everything from shaving mirrors to lighting the Olympic torch.

Now that you know the basic mirror types, let's learn about other, more unusual types of mirrors. Here's a short list:

• Non-reversing mirrors: Patents for non-reversing mirrors go back as far as 1887, when John Derby created one by placing two mirrors perpendicular to each other [source: O'Mara].
• Acoustic mirrors: Acoustic mirrors are huge concrete dishes built to reflect and distribute sound instead of light. The English military used them before the invention of radar as an early warning system against air attacks.
• Two-way mirrors: These mirrors are made by coating one side of a sheet of glass with a very thin, very lightly reflective material. When the coated side faces a lighted room, some of the light reflects and some goes into a dark room behind the mirror, making it possible to see into the lighted room but not out. (If you've ever watched an interrogation scene on a police show, you've seen one of these mirrors.) Glass is also a mildly reflective material -- this is the same reason that seeing outside at night is difficult if you turn on the lights in your house.