Meena's curved mirrors

On a summer holiday Meena went to a science centre with her family. While her parents looked at ways to save water and electricity, Meena and her brother found a row of strange, curved mirrors.

In one, her face looked huge, and her brother, standing farther back, looked upside down. In another, she looked tiny. Meena was puzzled: in Grade 7 (Light: Shadows and Reflections), a mirror always showed an image the same size as you, and the right way up.

A guide smiled. These aren't plane mirrors, he said. *They're spherical mirrors. When a mirror curves inwards or outwards, your image changes.*

Try it with a spoon (Activity 10.1). Hold a shiny steel spoon close to your face, move it slowly away, then turn it over and do it again.

Curved mirrors can be specially made too. A common kind is the spherical mirror, shaped like a piece of a hollow glass ball. If its shiny (reflecting) surface curves inwards, it's a concave mirror. If it curves outwards, it's a convex mirror. In drawings, the back (the side that doesn't reflect) is shaded.

Tell them apart (Activity 10.2). Lay a concave and a convex mirror on a table, shiny side up, and look at them from the side with your eye at table level. You can see which way each one curves.

Real spherical mirrors aren't cut from a glass ball. A flat piece of glass is ground and polished into a curve, and a thin shiny coating such as aluminium is put on: on the outer curved surface for a concave mirror, on the inner one for a convex mirror.