10.4 Converging & Diverging
Every ray on a curved mirror still obeys i = r. But because the surface is curved, the family of parallel rays does something a flat mirror never could.
Activity 10.6 — Many beams at once Send several parallel beams (through the teeth of a comb) at a plane, a concave, and a convex mirror in turn:
- Plane mirror: the reflected beams stay parallel.
- Concave mirror: the reflected beams come together — they converge.
- Convex mirror: the reflected beams spread apart — they diverge.
The Big Difference - A concave mirror converges a parallel beam (brings the light together). - A convex mirror diverges a parallel beam (spreads the light out).
Each individual ray still follows the laws of reflection — it’s the curved shape that makes the whole beam converge or diverge.
Light hot enough to burn
Activity 10.7 — A concave mirror that burns paper Face a concave mirror at the Sun and aim the reflected light onto paper, adjusting until you get a tiny bright spot. Hold it steady — the paper heats up and can begin to smoke and burn! The mirror has concentrated the Sun’s parallel rays onto one point, packing lots of energy into a small area. (Never look at the Sun or into the mirror — focus only on the paper, under adult supervision.)
A Step Further: Solar Concentrators Devices that focus sunlight with mirrors (and lenses) are solar concentrators. The concentrated heat boils a liquid to make steam — to generate electricity, for large-scale cooking, or in solar furnaces so hot they can even melt steel (recall the electric furnace from Chapter 4!). The same converging trick, scaled up to power a village.
Coming Up Mirrors reflect light to converge or diverge it. But there’s another way to bend light — by passing it through a curved transparent material. That’s a lens, and it can converge or diverge light too. Let’s look through one.