← Ray Model of Light

Lesson 3 of 8 · Textbook 5.2

Plane mirrors and their images

Five characteristics, and you will be asked for them by name. Three are obvious, and two are the ones that cost marks.

Where the image comes from

A plane mirror has a reflecting surface that is smooth and flat. A still body of water on a non-windy day forms the same kind of surface, which is why you see the sky and the trees in a reservoir.

The five characteristics

You and your image

plane mirror you image
Your distance—
Image distance—

Move the slider and watch the two distances stay equal. Then raise the right hand and watch which hand the image raises.
Learn all five, in these words 1. The image is the same size as the object.
2. The image is upright.
3. The image is virtual — formed inside the mirror and cannot be formed on a screen.
4. The image is laterally inverted — the mirror reverses left and right.
5. The image is the same distance from the mirror as the object is.
What “screen” means here A screen does not mean a projector screen only. It can be any physical object — a wall, a piece of paper — that provides a surface for an image to be formed on. Saying an image is virtual means you could hold a piece of paper anywhere behind the mirror and nothing would appear on it.

Lateral inversion, and the ambulance

If you raise your right hand, your image appears to raise its left hand. That is lateral inversion.

It is also why AMBULANCE is printed in reverse on the front of an emergency vehicle. A driver ahead sees it in their rear-view mirror, and the mirror laterally inverts it back — so the reversed word reads correctly, and the driver knows at once to give way.

Uses of plane mirrors

UseWhich characteristic is being used
Checking your appearanceThe image is the same size and upright, so you see yourself as you are.
Lining the walls of a buildingThe images make the interior appear wider than it is.
A periscopeTwo plane mirrors let navy officers in a submarine see above the surface of the sea.
An eye chartA mirror reflects the chart so it appears further away than it actually is — useful in a small consulting room.
A virtual mirror A virtual mirror uses augmented reality to generate an image of a shopper on a screen that behaves like a mirror, letting them try on clothes without undressing. Because the image on the screen is the same size as the shopper, they can choose clothes of the right size.

Check yourself