To make a mirror, all you really need is a smooth, polished surface. When light bounces off a smooth surface, it doesn't scatter, and you can see an undisturbed image. Before the Middle Ages, people polished obsidian or metals like copper, bronze, silver, gold and even lead. Later, people learned to place glass in front of the polished metal to produce a clearer image. Today, mirrors are smaller, lighter and more affordable, thanks to a process called silvering, which entails spraying a thin layer of silver or aluminum directly onto the back of the sheet of glass.
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