What Is a Plane Mirror?

In the study of physics, concave and convex mirrors are often discussed and experimented with – but not nearly as much attention is given to the type of mirrors used every day. A mirror with a flat surface is a plane mirror: A "standard" mirror without an inward or outward curve. These mirrors can be found almost anywhere – from bathrooms to hallways to building exteriors – and knowing how they reflect light can make understanding the more complex mirror variants significantly easier.

TL;DR (Too Long; Didn't Read)

A plane mirror is a flat mirror that reflects light and produces a virtual image without the interference of an inward or outward curve. Plane mirrors, which include the common bathroom and hallway mirrors used daily, produce a virtual image at the same magnification and distance as the object they reflect.

Mirrors at Home

Plane mirrors are simply flat mirrors without curves. Because these can be found almost anywhere, the average person is incredibly familiar with them (even if they don't know the technical term). While the first manmade mirrors were made from intensely polished bronze, silver and other metals, today most mirrors are made from glass sheets finished with a thin layer of aluminum. That said, plane mirrors can be made from liquid as well: Gallium and mercury can be used for this purpose. Regardless of material construction, however, all flat mirrors function the same way. They reflect rays of light, producing an image.

Real vs. Virtual Image

The images reflected by a plane mirror are known as "virtual images" – but they're different than the simulated digital images you can see on your computer screen or in a video game. In physics, the difference between a real vs. virtual image is that a real image is formed when light converges at a point – like when you look at an apple on your desk – whereas a virtual image is formed from two divergent rays of light that never meet. In simple terms, a plane mirror creates an image of an object you cannot touch. All mirrors create virtual images in this manner, but plane mirrors reflect light differently than concave or convex mirrors do.

Plane Mirror Reflections

Because a plane mirror is flat, light reflected off if it does not curve inward or outward. As a result, images from a plane mirror lack the interference seen from concave and convex mirrors. This is why you don't look strange when examining your hair in a bathroom mirror, but your body looks too tall or too short when looking at a curved mirror at a carnival. Plane mirror reflections create upright virtual images at the same magnification, or size, and distance as the objects they reflect. This is why you can use a plane mirror to determine exactly where something behind you is.

Convex and Concave Mirrors

In contrast to plane mirrors, convex and concave mirrors curve the rays of light that hit them. This results in the virtual images produced by their reflections coming out distorted, as the light rays move towards or away from the center of the mirror. For this reason, convex and concave mirrors aren't useful in bathrooms, but they can be helpful in the right situation; for example, because plane mirrors can't produce useful images at certain angles, the mirrors on the side of a car are convex. They allow drivers to see behind and to the sides of their vehicle, though these virtual images aren't at the same distance as the objects they reflect. This is why car mirrors have messages reminding drivers that objects in the mirror may be closer than they appear in the reflection.