Reflecting on mirrors

Since ancient humans first polished reflective stone, mirrors have played a sizable role in history. Mirrors feature prominently in fairy tales, science fiction, and fantasy stories, but they’ve also helped us gaze at distant planets, moons, and stars while also aiding in our exploration of the subatomic world. Mirrors form the very foundations of today’s most advanced technologies, and, of course, they play a role in how we perceive ourselves. 

The very concept of a mirror predates the technology itself, as ancient hominins likely stared at their reflections in the still waters of pools, ponds, and lakes. But as our understanding of mirrors has grown, so has their usefulness. It’s not a stretch to say the modern world wouldn’t be possible with the humble yet fascinating mirror.

Early ones weren’t made of glass

Although mirrors work thanks to complicated physics and a dash of quantum mechanics, these objects have actually been around for thousands of years. Crystal-clear water notwithstanding, the first mirrors were likely made in Anatolia (modern-day Turkey) around 8,000 years ago. However, these mirrors were likely made of obsidian, a kind of reflective volcanic glass, and in the following millennia, artisans dabbled with using different polished metals, such as bronze, copper, tin, and silver. That last element and cheap aluminum, which was introduced toward the end of the 19th century, became the metals of choice for their high reflectivity, meaning they reflect nearly all light back to the viewer. 

Of course, a white piece of paper also reflects all light in the visible spectrum, so why aren’t mirrors white? Due to its ultra-smooth surface, a mirror produces a “specular reflection.” This means the angle at which light hits the mirror (known as the angle of incidence) forms an equal angle when the light is reflected (the angle of reflection). A white sheet of paper, on the other hand, has surface imperfections that create a “diffuse reflection,” meaning the light scatters in all directions, which explains why mirrors show a reflection and paper doesn’t.

Some factors can alter reflections, such as the shape of the mirror (think of funhouse mirrors), the materials used to make the mirror, as well as the surrounding atmosphere. But under normal conditions, your mirror follows the above sequence of optical physics, known more broadly as the law of reflection.

To achieve this reflective physics in modern mirrors, a sheet of glass is cleaned and polished to remove impurities and then a coating of silver or aluminum is applied along with a chemical activator so the metal of choice bonds with the glass. Light enters through the glass, then reflects off the silver and aluminum and back to you, so you can finally check your hair or see if anything is stuck in your teeth.

Don’t like how you look in photos? Blame the mirrors

Mirrors don’t actually reproduce the exact version of you that other people see. Imagine standing in front of your reflection: The mirror doesn’t flip you horizontally (along the y-axis) as you might expect, but it does flip your image along the z-axis, which represents depth. This z-axis inversion is a bit like turning a glove inside out. This is why words on a T-shirt appear backwards in a mirror. 

This phenomenon is why many people don’t like how they look in photos. Because no one’s face is perfectly symmetrical, there are subtle changes between the face you see in the mirror and the face a camera captures on its sensor. However, when you take a selfie on a smartphone, the camera flips the image like a mirror would, so it appears more natural or familiar to us. Once you’ve taken the photo, the image that’s saved to your device is inverted so the final product looks like how other people (and the camera lens) see you. 

Mirrors are surprisingly high-tech

Mirrors have often been associated with vanity (think of the Evil Queen in Snow White), but in reality, their usefulness far exceeds self-admiration. In fact, mirrors are the backbone of many scientific fields. NASA’s James Webb Space Telescope — the most advanced space telescope ever made — uses an array of mirrors, or reflectors, to gather light and create stunning images of the universe. Many detectors at the CERN particle accelerator in Switzerland, the laboratory responsible for breakthrough discoveries such as the Higgs boson, use asystem of mirrors for reflecting light. Oh, and microscopes? Yeah, we have mirrors to thank for them too.

Then there are lasers, which rely on mirrors for building up and manipulating a focused beam of light. In December 2022, the National Ignition Facility at Lawrence Livermore National Laboratory in California used a complicated series of lasers and mirrors to achieve the first net-gain fusion reaction — the same physics that powers the sun — in hopes of one day providing limitless clean energy. So not only have mirrors helped form parts of today’s world, they’re also at the forefront of tomorrow.