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 appears to be shifted from left to right (we say it’s mirrored, but scientists say it’s “laterally inverted”). Lots of people find this very confusing, and I explain why it happens in the box below.
- 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).