_____ types of mirror are often used to concentrate sunlight to produce heat in solar furnace.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Concave
A concave mirror is a converging mirror: rays of sunlight falling on it parallel to its axis are reflected to meet at its focus. All the sunlight collected by a large mirror is thus concentrated on a small spot, which becomes very hot; solar furnaces and solar cookers use such mirrors.
Why the other options are incorrect
Option (2) Convex — Incorrect. A convex mirror spreads light out; it is used as a rear-view mirror in vehicles.
Option (3) Transparent — Incorrect. A transparent material lets light pass through and is not a type of mirror.
Option (4) Opaque — Incorrect. Opaque only means light cannot pass through; it says nothing about focusing it.