How mirrors are made
The conventional modern mirror is usually nothing more than a sheet of glass attached to a thin layer of metallic backing. It seems as if mirrors have been around forever in some form or another, but mirrors as we know them today haven’t been around that long. As early as a thousand years ago, mirrors were still polished discs of plain metal that cost more than most people of that era could afford: A peasant who wanted to see his or her reflection had to go look in a pond like everyone else — and had to stand in line to do it. Full-length mirrors are an even more recent invention. They’re only about 400 years old.
You’d think that four centuries would give people time to adjust to looking at themselves, but you’d have another thing coming. In a 2005 study at the University of Liverpool, a group of researchers asked subjects to predict when their reflection would appear as they walked past a mirror. Their answers were embarrassingly off. The same poor results came back when people were asked to judge the size of their heads in the mirror
The results of the Liverpool study suggest that humans simply aren’t intuitively equipped to deal with reflections, yet mirrors resonate deeply in the human psyche. They represent truth and illusion at the same time. They show us ourselves as we are — yet not quite — and we see a new world to explore behind the mirror that we can’t access. Perhaps these disorienting paradoxes are what make mirrors so central to both magic and science.
How mirrors work
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. 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. Mirrors are now used for all kinds of purposes, from LCD projection to lasers and car headlights.