Mirror
Perhaps the most common optical instrument in the world, mirrors have been used for millennia; artifacts made of polished obsidian dating back 7,500 years have been found in Turkey. The earliest man-made mirrors, which were constructed of highly buffed copper, brass, and bronze, were manufactured in the first century A.D. Not until the thirteenth century did the Venetians develop a method for silvering glass to make it more reflective. The modern silvering process was invented in 1835 by the German chemist Justus von Liebig, and, with a few improvements, it is his process that we presently use to manufacture mirrors today. Technically, a mirror is any smooth surface that reflects more light than it absorbs. The mirrors with which we are familiar are actually panes of glass that have one side coated with a very thin layer of metal, which may subsequently be coated with another layer of dielectric film. The amount of light reflected by a mirror is dependent upon the kind and quality of the materials used in its construction. No matter how skillfully crafted, no mirror can reflect all the light shone upon it.
There are two basic classes of mirror: planar (flat) and non-planar (curved). In the case of a planar mirror, the light that strikes the surface will be reflected away at a precise angle; the angle at which it leaves the mirror, called the angle of reflection, is equal but opposite to the angle at which it arrives, called the angle of incidence. The image seen in a planar mirror, called the virtual image , is an illusion; since it appears that the subject in the mirror is behind the surface, at a distance exactly twice the actual distance. For example, a mirror five feet away creates a virtual image ten feet distant; actually, the light does travel ten feet--five to the mirror and five back to the observer's eye. Also, the image in a planar mirror is always reversed: the right hand becomes the left hand, and writing appears backwards.
This reversal can be corrected by adding a second mirror, essentially reversing the image twice. This is the case with a periscope, which uses two mirrors to see around corners or above obstacles.
There are two major types of non-planar mirrors: concave and convex. Concave mirrors appear bowed, curving toward the observer like the outside of a sphere. The image viewed in a concave mirror appears smaller than it would in a planar mirror, but the field of view is greater. This type of mirror is often used in automobiles as side mirrors; usually bearing a warning that objects are closer than they appear, these mirrors allow the driver to see more of the road than would a simple flat mirror. In addition, drivers can now adjust automobile mirrors from inside the car, a useful safety feature. Convex mirrors are curved away from the observer, like the inside of a sphere. Mirrors with this shape can focus a path of light to a central point or expand a point of light into a wide path. Also found in automobiles, convex mirrors are used in headlights to turn the light emitted from a bright bulb into a powerful beam (a typical science class demonstration involves a device called a solar cooker; consisting primarily of a large convex mirror, the solar cooker can burn paper or even heat a hot dog by focusing a wide area of sunlight to a point). Concave and convex mirrors can also be used in place of magnifying lenses. Other curved mirrors shaped like parabolas, hyperbolas, and ellipses are manufactured for use in scientific instruments, such as reflecting telescopes. Mirrors are also used in homes for decorative purposes, enlarging the apparent appearance of a room.
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