Curved mirrors and parallel rays
A torch has a curved mirror behind its bulb. Why does the beam stay narrow? Parallel rays stay the same distance apart.
- Plane mirror: parallel rays stay parallel.
- Concave mirror: the rays come together.
- Convex mirror: the rays spread apart.
The rule
The beam is narrow because its rays are parallel. Each ray still follows the law of reflection. The curve changes which way the normal points.
Parallel rays stay parallel after a plane mirror. A concave mirror brings them together. A convex mirror spreads them apart.
A concave mirror brings parallel rays to one spot, its meeting point.
In a torch, the bulb sits at the mirror's meeting point. Light from that spot leaves in parallel rays. That makes a narrow beam.
| Mirror | Use |
|---|---|
| Concave | Torch reflector, dentist's mirror |
| Convex | Side-view mirror, road-bend mirror, shop mirror |
A convex mirror shows a wider area of the road behind.
Watch out!
Never point a concave mirror at the Sun or at anyone's eyes. The mirror gathers light into a hot spot.
Pick a mirror. Watch the five rays.
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