Mirrors—the science of reflection
by Chris Woodford. Last updated: August 7, 2011.
Humans spend hours preening themselves in mirrors and, given half the chance, apes and dolphins would do too. Peacocks are a different kettle of fish entirely. Park a shiny blue car near one of these pesky hooting birds and it'll peck and beat the panels to a pulp, convinced it's looking at a rival or a mate! Reflections in mirrors are amazing things that tell us literally (and psychologically) a great deal about how we see ourselves. But what do reflections tell us about the mirrors themselves? What exactly is a mirror... and how does it work?
Photo: These 16 mirrors are focusing sunlight onto a Stirling engine, which is being used to generate electricity at the Arizona Public Service Solar Test and Research Center. Photo courtesy of US Department of Energy.
Energy can never disappear
If you're a fan of recycling, you'll love one of the most fundamental scientific laws in the universe called the conservation of energy. The basic idea is really simple: you can't make energy out of thin air or throw it away. It doesn't matter what you do or how hard you try, you can never create energy or destroy it: the best you can do is to convert it into a different form—recycle it, if you prefer.
Think what happens when you switch on an electric lamp, you're not creating light from nothing. The light is being made from electricity flowing into your home from a power plant. But even a power plant doesn't make energy: it extracts energy locked in a fuel such as oil or coal and turns it into electricity. How did the energy get into the coal? It originally came from the Sun. Where did the Sun get the energy? From nuclear reactions happening deep inside its core. And so on...
Picture: Energy always has to come from somewhere... and go somewhere. This bubble-shaped explosion of gas and dust is 14 light-years wide and is expanding at 4 million miles per hour (2,000 kilometers per second). It's the remnant of a supernova (exploding star). Photo courtesy of NASA Jet Propulsion Laboratory (NASA-JPL).
No matter what you do, the conservation of energy is always looking over your shoulder. You need energy to do things, but the energy has to come from somewhere to begin with and go somewhere else when you're done. If you kick a football, potential energy stored in your muscles is transferred into the ball and makes it fly through the air with kinetic energy. When the ball rolls over the ground, friction (the rubbing force between the ball and whatever it touches) steals away its energy and brings it to rest.
The conservation of energy is the reason why you can't build a machine that will run by itself forever (known as a perpetual motion machine). It's also the reason why your car inevitably runs out of petrol and why you always get an electricity bill coming through your letterbox every few weeks.
What happens when light hits a mirror?
What's all this got to do with mirrors? When you stand in front of a mirror, what you see is the conservation of energy in action, working its magic on light. Light is energy traveling at high speed (300,000 km or 186,000 miles per second) and, when it hits an object, all that energy has to go somewhere. There are three things that can happen when light hits something: it can pass through (if the object is transparent), sink in and disappear (if the object is opaque and darkly colored), or it can reflect back again (if the object is shiny, light-colored, and reflective). Either way, the Conservation of Energy is at work: there is just as much energy around before light hits something as afterward, though some of the light may be converted into other forms.
What happens when you look in a mirror? In the daytime, light reflects off your body in all directions. That's why you can see yourself and other people can see you. Your skin and the clothes you're wearing reflect light in a diffuse way: light rays bounce off randomly, haphazardly, in no particular direction. Stand in front of a mirror and some of this light from your body will stream in straight lines toward it. Rays of light (which are really packets of light energy called photons, fired in a stream like bullets from a machine gun) shoot through the glass and hit the silver coating behind it (possibly a real coating of silver or more likely something less expensive such as polished aluminum). The light will reflect off the mirror in a more orderly way than it reflects off your clothes. We call that specular reflection—it's the opposite to diffuse reflection.
How does the mirror reflect light? The silver atoms behind the glass absorb the photons of incoming light energy and become excited. But that makes them unstable, so they try to become stable again by getting rid of the extra energy—and they do that by giving off some more photons. (You can read about how atoms take in and give out photons in our article about light.) In practice, the back of a mirror is usually painted black, covered with wood, or pressed against a wall, so as much light is reflected as possible. Silver reflects light better than almost anything else and that's because it gives off almost as many photons of light as fall on it in the first place. The photons that come out of the mirror are pretty much the same as the ones that go into it.
Artwork: How a mirror works: silver atoms inside catch and reflect light beams. For the conservation of energy to hold true, light beams must reflect back at the same angle they hit the mirror. The black backboard stops light beams from penetrating through the glass, increasing its ability to reflect.
What different types of mirrors are there?
Have you ever been in a hall of mirrors at a fun fair? If so, you'll have seen amazingly distorted reflections of yourself looking short and fat or tall and skinny. In the words of the old cliché, it's all done with mirrors. What you see when you look at a mirror is not what's really there but what your brain thinks is there based on how it thinks the image is being created. In other words, what you see is an optical illusion. Even the image in a normal mirror is an optical illusion, because you're not really standing eight feet in front of yourself grinning back. What you see is a virtual image, not a real object.
Broadly speaking, there are three types of mirror:
- If the surface of a mirror is perfectly flat (what's known as a plane mirror), what you see in the glass is a reasonable approximation to what's really there—but with one crucial difference: the image is shifted from left to right (we say it's mirrored, but scientists say it's "laterally inverted").
- If the mirror bows inward at the center (known as a converging mirror or concave mirror), light rays will appear to come from in front of the mirror, the reflection will be nearer to you, and reflections will appear bigger than they really are. That's why a converging mirror magnifies. Shaving mirrors work like this.
- In a mirror that bulges outward at the center (a diverging mirror or convex mirror), the opposite happens. Light rays seem to come from behind the mirror and reflections will appear smaller and further away than they would in a plane mirror. Driving mirrors work like this (and so does the back of a spoon if you hold it just right).
Photo: Two kinds of mirrors that work in opposite ways.
Left: A shaving mirror is a converging mirror.
Right: A spoon is a very imperfect diverging mirror. All the scratches
in its surface mean it reflects less perfectly than a mirror but better than most random
pieces of metal.
Why do polished objects shine like mirrors?
You can make things more mirror-like by polishing them. If something is flat and light-colored, you can make it reflect light better by rubbing it clean. What you do when you polish is a combination of rubbing away some dirt, filling in bumps and scratches, and adding a surface layer of a chemical such as wax, all of which make the surface smoother and more like a mirror. When light hits, it's more likely to be reflected back in an orderly, specular fashion with parallel incoming rays of light reflected back again as parallel outgoing rays of light. That's why people talk about polishing a pair of shoes until you can "see your face in them."
Photo: One good reason to polish your GORE-TEX® boots: you can think about the science of light reflection and the conservation of energy as you're doing it.
Of course, if you're polishing something like a car, you have a big advantage: it's made of flat metal, smooth, and often light-colored. So removing the dirt and rubbing the surface smooth is really all you need to do to make it shine. Some polishes contain optical brighteners to trick you into thinking things are cleaner and shinier than they really are. These chemicals are widely used in washing detergents and appear to reflect more light back than they actually absorb. That's not a violation of the conservation of energy—quite the opposite: the atoms inside optical brighteners absorb and re-emit the light that falls on them in such a way that energy is precisely conserved. They do this by absorbing invisible ultraviolet light (the blueish light in sunlight that our eyes can't see) and converting it into a blue light we can see. So as well as reflecting the light we can see, they're reflecting light we can't see and turning it into light we can—and that's how they make things seem brighter without violating the laws of physics.
Mirrors in space
Imagine gazing into a mirror and seeing not your face but distant stars and galaxies. The world's biggest telescopes work in exactly the same way as the mirrors on your walls at home, only they are more precise, point toward the sky, and are much bigger. There is a limit to how big a mirror can be made (typically around 8m or 27 ft across) without buckling under its own weight. Some telescopes have bigger mirrors made from smaller pieces joined together. The two Keck telescopes on Mauna Kea mountain in Hawaii have mirrors 10m (33 ft) across, each one made from 36 separate mirror tiles. At home or in space, mirrors must have completely smooth surfaces to make undistorted images. Making space mirrors involves particularly laborious and elaborate polishing, as this great photo of the Hubble Space Telescope's mirror being polished shows you very clearly. Next time you have to spend a little while polishing a mirror at home, spare a thought for the optical scientists who manufacture space mirrors: they can take months (and sometimes years) to buff up to perfection!
Photo: Large mirrors are difficult to make, so giant space mirrors like this one are often made from multiple, separate hexagonal elements fitted together in a honeycomb pattern. This mirror is about 5m (18ft) in diameter and made from precision fabricated aluminum segments. Photo courtesy of NASA Marshall Space Flight Center (NASA-MSFC).
Further reading
On this website
Other websites
- History of Mirrors Dating Back 8000 Years: by Jay Enoch. Optometry & Vision Science: October 2006, Volume 83 Issue 10, pp 775-781. Covers "the ancient history and early development of mirrors."
- Corning Museum of Glass: Telescopes and Mirrors: Read about some of the world's most amazing mirrors destined for space telescopes—how big they are and how long they took to manufacture!
Books
For younger readers
- Eyewitness: Light: by David Burnie, Dorling Kindersley, 2000. A good introduction for ages 9-12 covering both the theory, practical uses, and history of light science.
For older readers
- Optics: by Eugene Hecht, Addison Wesley, 2002. Still a great college textbook (and the one I used myself as an undergraduate).
