Images in a plane mirror

Look in a mirror and you see your face. What you see is a reflection. Activity 11.7 studies it with a pen.

Stand a pen in front of a plane mirror. It looks as if a matching pen is standing behind the mirror. That pen 'behind' the mirror is the image; the real pen is the object.

Move the pen around and compare. The image in a plane mirror is always the same size as the object — not bigger, not smaller.

Is the tip of the pen still at the top in the image? Yes, every time. An image that is the right way up is called erect. A plane mirror's image is erect.

Now hold a screen behind the mirror, and in front of it, and move it about. You cannot catch the image on a screen; it only appears when you look into the mirror.

Activity 11.8 uses your own reflection. Stand close to a mirror: your image looks close. Step back: it looks far away.

Now raise your left arm. Your image seems to raise its right. Touch your right ear, and your image touches its left. This swap of left and right is called lateral inversion. That is why AMBULANCE is written back-to-front on the front of an ambulance: in the rear-view mirror of the car ahead, it reads the right way round.

Early mirrors were made by polishing stone or metal. Once glass mirrors arrived, metal mirror-making was mostly forgotten, but it survives in Kerala, where the Aranmula Kannadi, a special metal mirror, has been made for centuries.