How Does a Mirror Work?

May 8, 2018

By Jordan Scott

When you look into a mirror, you see your own image and your surroundings reflected back at you. But how many times have you tried to look at the mirror itself, and question how it works?

If the answer is at least once, know you’re not alone. To figure out how a mirror works, it’s best to begin with how it’s made.

There are so many mirror options available now that the possibilities are nearly endless. No matter what mirror you need for your home, it can likely be built.

Making a Mirror

We’ve come a long way since water was the only medium through which to see a reflection. In ancient times, mirrors were made from expensive metals such as bronze and silver.

Now, mirrors are most often made by covering flat glass with a reflective coating, such as silver. Glass provides a solid base for mirrors due to its smooth surface and rigidity. Plus, it’s relatively easy to make.

The glass used must be polished, and without imperfections. Special techniques are used to make curved mirrors.

The first step in the process is cutting the glass to shape and size, which can vary greatly depending on application. Once the glass is the correct shape, size and smoothness, the reflective coating is applied in an evaporator, or large vacuum chamber. The metal is heated to the point that it evaporates, depositing a coating onto the glass surface.

Types of Mirrors

There are several types of mirrors in use around the world.

Plane mirrors – These are mirrors with a flat, reflective surface. Plane mirrors reflect an image proportionally. This type of mirror is used most commonly in bathrooms and bedrooms to reflect a person’s image. It can also be used to make a room appear larger and brighter.
Convex mirrors – These are spherical mirrors that bulge out, like the bottom of a spoon, distorting the image by making it appear smaller. Convex mirrors are often used for automotive side mirrors to allow drivers to see a greater area behind them.
Concave mirrors – These are spherical mirrors that curve inward, like the top of a spoon. The make an image appear larger and are often used for security and medical purposes, telescopes and behind car headlights to focus and brighten the light.
Two-way mirrors – These mirrors have a lightly reflective coating on one side of the glass. The mirror will look like normal glass if the amount of light is the same on both sides of the glass, but will appear as a mirror to people on the brighter side if one side is bright and one is dark. A similar phenomenon occurs when you look outside of a window at night. You can’t see out very well but people can see you easily.

Types of Reflection

Specular reflection – A highly-polished surface such as a man-made mirror is able to create a specular, or precise, reflection that appears to be an inverted version of the actual object being reflected. The light bounces off the mirror straight back in the same direction it just came from.
Diffuse reflection – Water causes a fuzzy reflection because it is not perfectly smooth like a man-made mirror. The light does not bounce off the water precisely but scatters, bouncing off the water from the same angles it hit the uneven water. This results in a blurred reflection.

city reflection in water — *Diffuse reflection in* water.

The Science of Reflection

Light is energy. When that energy hits an object is can pass through the object, be reflected or be absorbed. The amount of energy remains the same no matter the outcome. Mirrors reflect light energy. Initially, the atoms that make up the metallic coating absorb the energy, but they are not able to maintain equilibrium and become unstable, resulting in the light energy being expelled.

Most people believe that when they look into a mirror, it laterally inverts their image but if mirrors invert horizontally, why don’t they invert vertically, causing that person’s image to appear upside down? If you write a non-symmetric letter on clear plastic and hold that up to a mirror, the letter will appear normal to you, not inverted in any direction. That’s because the mirror reflects light back the same way it receives it. If you wear a shirt with words on it and stand in front of a mirror, the words on the reflected shirt will appear inverted. That’s because the shirt is facing away from you while you’re wearing it. If you held that same shirt in your hands and it magically became transparent except for the words, the words’ reflection would appear normal to you in a mirror.

However, that doesn’t mean that mirrors don’t invert an object’s reflection at all. They do. Rather than inverting left to right or up and down, mirrors invert a reflection perpendicularly. If you touch a business card to a mirror so that the back of the card is parallel with the ground and the left side of the card is touching the mirror, the text will appear inverted.