Converging and diverging mirrors

The laws of reflection work for every mirror: plane, concave and convex. But send many parallel rays at a curved mirror and something interesting happens.

Activity 10.6. Use the same set-up, but leave many gaps in the comb uncovered, so you get several parallel beams. Shine them onto a plane, a concave and a convex mirror in turn.

That's why a concave mirror is also called a converging mirror and a convex mirror a diverging mirror.

Every single ray still obeys the laws of reflection exactly. It's the curved surface that makes the difference: because the mirror curves, each ray meets it at a slightly different angle, and the beam as a whole comes together or spreads out.

Can a concave mirror gather enough light to matter? (Activity 10.7) Only with a teacher or adult. Never look at the Sun or into the mirror reflecting it, and point the bright spot only at paper, never at anyone. Face a concave mirror towards the Sun and aim the reflected light onto thin paper. Move it until you get a small, sharp, bright spot, and hold it still for a few minutes. The paper starts to smoke and burn, because the sunlight from the whole mirror is concentrated on one tiny spot.

Devices that gather sunlight into a small area using mirrors and lenses are called solar concentrators. The concentrated sunlight heats a liquid to make steam for generating electricity, cooks food on a large scale, or runs solar furnaces, which can even melt steel.