What if mirrors did not exist?

Look in the mirror. Who is that? It's you!

A mirror bounces light back to your eyes. What if there were no mirrors anywhere?

Every morning, a mirror shows you your face, your messy hair and your smile. But how does a mirror do that? Light bounces off you, then off the mirror, and back into your eyes.

Mirrors do far more than help us brush our hair. This lab is a thought experiment: what if there were no mirrors? We can't remove them all, but we can follow the clues.

OpenStax opens its treatment of reflection with three everyday cases: looking in a mirror, sunlight glinting off a lake, and large telescopes that use reflection to form images of stars. The governing rule is the law of reflection: the angle of reflection equals the angle of incidence.

This thought experiment removes mirrors, meaning smooth, shiny surfaces made to reflect, and asks what would change for self-image, astronomy and seeing around obstacles.

A mirror is smooth and shiny. It bounces light back neatly, so you see a clear picture.

Paper is rough. It bounces light every which way. You see the paper, but not your face in it.

All surfaces bounce some light. A mirror is so smooth that light bounces off it at the same angle it arrived, like a ball bouncing straight off a wall. That keeps the picture neat, so you see a clear image.

A sheet of paper is rough at a tiny scale. OpenStax explains that it scatters light in many different directions. That's why you can see the paper from any angle, but you can't see your face in it.

OpenStax distinguishes specular reflection from smooth surfaces, where parallel rays stay parallel and form images, from diffuse reflection from rough surfaces, where rays striking at different local angles leave in many directions. Diffuse light lets us see paper from any angle; moonlight on a lake is spread out because the surface is shiny but uneven.

A plane mirror's image appears behind it, exactly as far behind as you stand in front, because we see light arriving from a direction set by the law of reflection.

Imagine all the mirrors are gone.

You might not know if your hair was messy! Some telescopes might not work. And it might be hard to peek around corners.

Imagine every mirror vanished. You'd still see the world, because rough things scatter light. But you might rarely see your own face, except perhaps in still water or a shiny spoon. Getting ready in the morning could be tricky.

The bigger loss might be in science. OpenStax notes that large telescopes use reflection to form images of stars, and periscopes use mirrors to look around obstacles. Without mirrors, people might have to find other ways. These are ideas, built from how mirrors are used.

Without mirrors, ordinary vision might be largely untouched, since OpenStax notes that diffuse reflection is what lets us see people, clothing, leaves and walls from all sides, but images formed by specular reflection might be lost: plane mirrors, reflecting telescopes, mirror periscopes and retroreflectors.

Some functions might shift to other optics. Britannica notes that periscopes can use reflecting prisms instead of mirrors, and telescopes might use lenses, though NASA notes that Webb's 6.5 m mirror is the largest ever launched into space. How far substitutes could go is speculation; the losses described here are reasoned ideas.

Light bounces off a mirror like a ball bouncing off a wall.

Because the mirror is so smooth, all the light bounces the same neat way. That makes a clear picture.

Light follows a simple rule when it bounces: it leaves a surface at the same angle it arrived. On a mirror, every bit of the surface is tilted the same way, so all the light from your face bounces back together and keeps its pattern. Your eyes see that pattern as a picture of you, seeming to stand behind the glass.

On rough paper, each tiny bump is tilted differently, so the light scatters in all directions and the pattern is lost.

The law of reflection, θr = θi, measured from the surface normal, holds at every point of any surface. On a polished surface the normals are aligned, so rays from each point of an object keep their relative directions after reflection and an image forms. On a rough surface the normals vary randomly at small scales, so directional information is lost.

OpenStax also describes corner reflectors: two or three mutually perpendicular reflecting surfaces return light exactly back toward its source, whatever the angle of incidence.

The Hubble Space Telescope has a big mirror. It is wider than a grown-up is tall!

The Webb telescope has a golden mirror made of 18 pieces.

Some of the most important mirrors are in space. NASA's Hubble Space Telescope collects starlight with a mirror 2.4 meters across. Its mirror is so smooth that if it were as big as Earth, no bump on it would be taller than about 15 centimeters.

The James Webb Space Telescope has an even bigger mirror, 6.5 meters across, made of 18 six-sided pieces covered in a very thin layer of gold. On Earth, periscopes use two mirrors or prisms to let a person see while staying hidden or underwater, such as in a submarine.

NASA: Hubble's 828 kg, 2.4 m primary mirror is coated with aluminum and protected with magnesium fluoride, collects 40,000 times more light than the human eye, and works in a Cassegrain arrangement with a convex secondary mirror. Webb's primary is 6.5 m across, made of 18 hexagonal beryllium segments with a 100 nm gold coating that improves infrared reflection; a scaled-up Hubble-style mirror would have been too heavy to launch.

Britannica describes a periscope as an instrument with two mirrors or reflecting prisms that change the direction of the light, used in submarine navigation and elsewhere.