Ever since the first caveman saw himself in a pool of water and became the first human in history to complain of a bad hair day, we’ve recognized that being able to see our reflection has its uses–and so we’ve made mirrors.
A mirror is any object that reflects light rays in such a way as to form a virtual image. The mirror image is called “virtual” because, although it looks like, in our bathroom mirror, another person is standing in front of us, he or she isn’t really there–we’re just seeing the light rays bouncing off our own body reflected back into our eyes.
Early mirrors, such as those mentioned in the Bible, were made of polished brass, bronze or silver. Unfortunately, metal has a tendency to dull with age due to abrasion an oxidation.
A layer of glass can protect the reflective surface, but you’ve got to have good glass: light rays bouncing off the mirror will be refracted (bent) by any imperfections, resulting in a distorted image. Not surprisingly, Venice, renowned for the quality of its glass, produced the first glass-covered mirrors (by backing panes of glass with thin sheets of silver foil) in the 1300s.
In ensuing centuries, a new method came to the fore. Glass was overlaid with sheets of carefully smoothed tinfoil, then covered with mercury. A woolen cloth was laid over the surface and held down by iron weights for about day, then the excess mercury was drained away, leaving behind a reflective surface.
Modern mirrors don’t use mercury; insteady, they’re “silvered.” A solution of a silver-containing chemical such as silver nitrate is applied to the glass in such a way that a thin, highly reflective layer of pure metallic silver forms.
Mirrors used in telescopes and other optical equipment must meet higher standards than bathroom mirrors, and so are made by an entirely different process. Glass (or, sometimes, fused quartz, which doesn’t change size as much as glass when heated or cooled) is placed in a vacuum chamber, then the substance which will form the mirrored surface (typically aluminum, because it is almost as reflective as silver and doesn’t tarnish as easily) is vaporized by heating. The vaporized aluminum condenses out of the vacuum onto the glass, creating a very thin, near-perfect and highly-reflective coating.
The most reflective mirror humans have ever made is currently orbiting the Earth in the Hubble Space Telescope. This 2.4-metre mirror is so nearly perfectly smooth that, if it were the size of the Gulf of Mexico, no waves would be more than a millimetre high.
Even the best metallic mirror, however, absorbs a certain amount of light instead of reflecting. For some purposes, such as focusing a laser beam, that’s a serious problem. As well, metallic mirrors reflect almost all wavelengths of light; for some purposes, you only want to reflect specific wavelengths
Fortunately, there’s another kind of mirror that doesn’t share these shortcomings. “Dielectric” mirrors use materials that selectively conduct electricity, stacked in a layer. If they’re layered just right, they’ll reflect almost all of a particular wavelength or wavelengths. Until recently, unfortunately, they’ve had one big drawback: they only reflect light that hits them at specific angles. Researchers at MIT, however, think they’ve solved that problem–and as a result, dielectric mirrors may soon find new uses both inside and outside the laboratory.
By adjusting the type of materials in the mirror (one of which, believe it or not, is Styrofoam) and they way they’re stacked, they created a dielectric mirror that can reflect light from any angle. Not only that, it can be tuned to reflect only certain wavelengths–say, infrared (heat radiation) or ultraviolet. That means you could conceivably have a window in your car or home that would let all of the visible sunlight shine in without any accompanying heat or upholstery-fading ultraviolet. Jonathan Dowling, a research scientist at NASA’s Jet Propulsion Laboratory, predicts such windows could become commonplace in less than five years. NASA would love to have them, because they’d be of great use in the space shuttle as window coatings.
Of course, I can think of even better possible uses. How about a bathroom mirror that reflects everything except the wavelengths of light that bounce off of gray hairs and fat cells?
Alas, that calls for another type of mirror–the magic mirror. Even at MIT, they haven’t managed to invent one of those yet.
