Mirrors and Lenses

Summary

Mirrors and lenses are tools used to manipulate the direction of light and images, mirrors by using flat or curved glass in the reversion, diversion, or formation of images, and lenses by using polished material, usually glass, in the refraction of light. Scientific applications of mirrors and lenses cover a broad spectrum, ranging from photography, astronomy, and medicine to electronics, transportation, and energy conservation. Without mirrors and lenses, it would be impossible to preserve memories in a photograph, view far-away galaxies through a telescope, diagnose diseases through a microscope, or create energy using solar panels. Miniaturized mirrors are also implemented in scanners used in numerous electronic devices, such as copying machines, bar-code readers, compact disc players, and video recorders.

Definition and Basic Principles

Mirrors and lenses are instruments used to reflect or refract light. Mirrors use a smooth, polished surface to revert or direct an image. Lenses use a piece of smooth, transparent material, usually glass or plastic, to converge or diverge an image. Because light beams that strike dark or mottled surfaces are absorbed, mirrors must be highly polished in order to reflect light effectively. Likewise, since rough surfaces diffuse light rays in many different directions, mirror surfaces must be exceedingly smooth to reflect light in one direction. Light that hits a mirrored surface at a particular angle will always bounce off that mirrored surface at an exactly corresponding angle, thereby allowing mirrors to be used to direct images in an extremely precise manner.

Images are reflected according to one of three main types of mirrors: plane, convex, and concave. Plane mirrors are flat surfaces and reflect a full-size upright image directly back to the viewer but left and right are inverted. Convex mirrors are curved slightly outward and reflect back a slightly smaller upright image, but in a wider angled view, and left and right are inverted. Concave mirrors are curved slightly inward and reflect back an image that may be larger or smaller, depending on the distance from the object, and the image may be right-side up or upside down.

Background and History

The ancient Egyptians used mirrors to reflect light into the dark tombs of the pyramids so that workers could see. The Assyrians used the first known lens, the Nimrud lens, which was made of rock crystal, 3,000 years ago in their work, either as a magnifying glass or as a fire starter. Chinese artisans in the Han dynasty created concave sacred mirrors designed for igniting sacrificial fires, and Incan warriors wore a bracelet on their wrists containing a small mirror for the focusing of light to start fires.

It was not until the end of the thirteenth century, however, when an unknown Italian invented spectacles, that lenses subsequently became used in eyeglasses. Near the end of the sixteenth century, two spectacle makers inadvertently discovered that certain spectacle lenses placed inside a tube made objects that were nearby appear greatly enlarged. Dutch spectacle maker Zacharias Janssen and his son, Hans Janssen, invented the microscope in 1590. In 1608, Zacharias Janssen, in collaboration with Dutch spectacle makers Hans Lippershey and Jacob Metius, invented the first refractory telescope using lenses. However, it was Galileo Galilei who, in 1609, took the first rudimentary refracting telescope invented by the Dutch, drastically improved on its design, and went on to revolutionize history by using his new telescopic invention to observe that the Earth and other planets revolve around the Sun. In 1668, Sir Isaac Newton invented the first reflecting telescope, which, using mirrors, conveyed a vastly superior image than the refractory telescope, since it greatly reduced distortion and spherical aberrations often conveyed by the refractory telescope's lenses.

Almost two hundred years later, in 1839, with the invention of photography by Louis Daguerre, the convex lens became used for the first time in cameras. In 1893, Thomas Edison patented his motion-picture camera, the kinetoscope, which used lenses, and by the 1960s, mirrors were being used in satellites and to reflect lasers.

How It Works

The law of reflection states that the angle of incidence is equal to the angle of reflection, meaning that if light strikes a smooth mirrored surface at a 45-degree angle, it will also bounce off the mirror at a corresponding 45-degree angle. Whenever light hits an object, it may be reflected, or it may be absorbed, which is what happens when light hits a dark surface. Light may also hit a rough shiny surface, in which case the light will be reflected in many different directions, or diffused. Light may also simply pass through an object altogether if the material is transparent.

When light travels from one medium to another, such as air to water, the speed of the light slows down and refracts, or bends. Lenses take advantage of the fact that light bends when changing mediums by manipulating the light to serve a variety of purposes. The angle that light is refracted by a lens depends on the lens's shape, specifically its curvature. A glass or plastic lens can be polished or ground so that it gathers light toward its edges and directs it toward the center of the lens, in which case the lens is concave, and the light rays will be diverged. Conversely, a lens which is convex, because it is thicker in the middle and has thinner edges, will cause light rays to converge.

Both concave and convex lenses rely on the light rays' focal point either to magnify, reduce, or focus an image. The focal point is simply the precise point where light rays come together in a pinpoint and focus an image. The distance from the center of the lens to the exact point where light rays focus is called the lens's focal length. Light that passes through a convex lens is refracted so that the rays join and focus out in front of the lens, creating a convergence. Light rays passing through a concave lens are refracted outward and create a divergence because the focal point for the light rays appears to be originating from behind the lens. Aberrations such as blurriness or color distortion may sometimes occur if lenses are made using glass with impurities or air bubbles or if the lens is not ground or polished properly to make it precisely curved and smooth.

Applications and Products

Electronics. Lenses and mirrors are used extensively in consumer electronics, primarily as part of systems to read and write optical media such as CDs and DVDs. These optical disks encode information in microscopic grooves, which are read by a laser in much the same manner as a turntable's needle reads a record. Good-quality lenses are essential to focus the laser beam onto the disk and to capture the reflection from the disk's surface. Lenses, and sometimes mirrors, are used in car headlights and LED lights.

Cameras also make use of both mirrors and lenses. In single-lens reflex (SLR) cameras, the film or image sensor is protected from light by a flip mirror. When a photograph is taken, the flip mirror rotates, directing light onto the film or digital sensor to expose the image to a size that will fit onto the film. Modern camera lenses are actually composed of multiple simple lenses, which helps to improve the overall image quality.

Science. Astronomy has long been the driving force behind advances in optics, and many varieties of lenses and mirrors were developed specifically to enable astronomical observation. Large modern telescopes are limited not by the optical quality of their lenses and mirrors but by distortion caused by turbulence in the atmosphere. To address this, state-of-the-art telescopes use adaptive optical systems, which can change the shape of the telescope's primary mirror in response to fluctuations in the atmosphere, which are measured with a powerful laser.

Lens and mirror systems are also indispensable as tools in a larger research apparatus. Microscopes, high-speed cameras, and other digital equipment are commonplace in biological, chemical, and physical science laboratories, and many experimental setups include custom-made imaging equipment all based on combinations of mirrors and lenses.

Retail. Macy's, Bloomingdale's, Kohl's and other department stores have installed full-size digital touch-screen mirrors in their stores, revolutionizing the retail experience. The mirrors enable consumers to model clothes, makeup, and jewelry digitally, without actually having to try on the products. Because an extraordinary amount of time and energy is saved by the interactive retail mirrors, customers have more flexibility to shop creatively by sampling a broader range of merchandise. The interactive digital mirrors also offer those shopping for friends or relatives the added convenience of being able to gauge sizes correctly, greatly reducing the number of returns. The increased ease and efficiency of touch-screen digital mirrors has generated more customers and sales for both retailers.

Careers and Course Work

Courses in physics, astronomy, advanced mathematics, geometry, optical engineering, mechanical engineering, aerospace engineering, computer science, and chemistry provide background knowledge and training needed to pursue a future working with various applications utilizing mirrors and lenses. A bachelor's degree in any of the above fields would assist in the basic applications of mirrors and lenses, but careers involving research or development of lenses in the corporate domain or academia would almost certainly necessitate a graduate degree in one of the above disciplines—ideally a doctorate. Although employment at any university as an educator and researcher is one potential career path, with an emphasis in physics and astronomy, working at an observatory is also a career possibility. Having an additional concentration of course work in aerospace engineering is also outstanding preparation for employment with NASA and government space projects.

Additional career opportunities working with lenses abound in the medical field, especially as an optical engineer. Either a master's or doctorate in physics or optics is a prerequisite for a career as an optical engineer; however, an associate's degree is all that is necessary to become trained to work as a contact lens technician. To work as an optometrist, one must graduate from a college of optometry after completing a bachelor's degree, and to work as an ophthalmologist, one must obtain a medical degree.

Additional career opportunities in the “green” industry using mirrors in solar panels requires a background in environmental engineering, and work in the photographic industry requires extensive knowledge of photography as well as lenses.

Social Context and Future Prospects

Although the increased necessity of using mirrors in green technology (such as solar panels) has become universally recognized, mirrors may conceivably play a key role in another environmental technology in the future. Scientists researching the long-term impact of global warming, increasingly alarmed by the rapid escalation of carbon dioxide buildup in the Earth's atmosphere, have begun seriously investigating the potential for mirrors to help lower the temperature of the Earth's atmosphere. In the 2000s, scientists began studying the possibility of launching a series of satellites to orbit above the Earth, each one containing large mirrors, which could be controlled, like giant window blinds, to reflect back out into space as much of the Sun's rays as desired, thereby regulating the temperature of the Earth's atmosphere like a thermostat.

In the 2010s, mirrors and lenses were used to create smart mirrors: household mirrors integrated with touch screen technology that provide access to various applications—including social media, the weather, and news updates—through internet access. In the future, these mirrors may also perform other tasks, for example, monitoring an individual's health by analyzing a person's physical appearance. Vital signs, such as heart rate, blood pressure, and weight could be measured by the mirror and sent to a computer. Other mirrors may project a person's possible future appearance if certain negative lifestyle habits, such as smoking, overeating, excessive drinking, and lack of exercise, are practiced. Cameras placed throughout the home may record an individual's personal habits and then relay the information to an interactive home mirror linked to a computer. The mirror, in turn, may then portray possible physical results of a negative lifestyle to someone, traits such as obesity, discolored teeth, wrinkled skin, or receding hairline, turning mirrors into modern-day oracles.

Bibliography

Andersen, Geoff. The Telescope: Its History, Technology, and Future. Princeton, N.J.: Princeton University Press, 2007.

Burnett, D. Graham. Descartes and the Hyperbolic Quest: Lens Making Machines and Their Significance in the Seventeenth Century. Philadelphia: American Philosophical Society, 2005.

Conant, Robert Alan. Micromachined Mirrors. Norwell, Mass.: Kluwer Academic, 2003.

Kingslake, Rudolph. A History of the Photographic Lens. San Diego: Academic Press, 1989.

Komar, Paulina et al. "Experimental Demonstration of Light Focusing Enabled by Monolithic High-Contrast Grating Mirrors." ACS Publications, 19 May 2021, pubs.acs.org/doi/10.1021/acsami.1c04871. Accessed 10 June 2022.

Pendergrast, Mark. Mirror Mirror: A History of the Human Love Affair with Reflection. New York: Basic Books, 2003.

Zimmerman, Robert. The Universe in a Mirror: The Saga of the Hubble Space Telescope and the Visionaries Who Built It. Princeton, N.J.: Princeton University Press, 2008.