When humans started making simple mirrors around 600 B.C., they used polished obsidian as a reflective surface. Eventually, they started to produce more sophisticated mirrors made of copper, bronze, silver, gold and even lead. However, because of the weight of the material, these mirrors were tiny by our standards: They rarely measured more than 8 inches (20 centimeters) in diameter and were used mostly for decoration. One exception was the Pharos, the lighthouse of Alexandria, whose large metal mirror reflected sunlight during the day and the fire used to mark the lighthouse at night.
Contemporary mirrors did not come into being until the late Middle Ages, and even then their manufacture was difficult and expensive. One of the problems involved was the fact that the sand used for glassmaking contained too many impurities to produce real clarity. In addition, the shock caused by the heat of adding molten metal for backing almost always broke the glass.
It wasn't until the Renaissance, when the Florentines invented a process for making low-temperature lead backing, that modern mirrors made their debut. These mirrors were finally clear enough for artists to use. For example, architect Filippo Brunelleschi created linear perspective with a mirror to give the illusion of depth of field. In addition, mirrors helped jump-start a new form of art: the self-portrait. Later, the Venetians would conquer the mirror-making trade with their glass-making techniques. Their secrets were so precious and the trade so lucrative that renegade craftsmen who tried to sell their knowledge to foreign workshops were often assassinated.
At this point, mirrors were still only affordable for the rich, but scientists had noticed some alternative uses for them in the meantime. As early as the 1660s, mathematicians noted that mirrors could potentially be used in telescopes instead of lenses; James Bradley used this knowledge to build the first reflecting telescope in 1721 [source: Panek]. Despite the importance of this discovery, the fact remained that both were cost-prohibitive.
The modern mirror is made by silvering, or spraying a thin layer of silver or aluminum onto the back of a sheet of glass. Justus Von Leibig invented the process in 1835, but most mirrors are made today by heating aluminum in a vacuum, which then bonds to the cooler glass [source: Britannica]. Mirrors are now used for all kinds of purposes, from LCD projection to lasers and car headlights. But how do mirrors actually work? Find out on the next page.