SSC CGL 2025 Tier-I · 21 September 2025, Shift 1 · General Awareness · Question 19 of 99

We transcribed this text from the pictures on a candidate's response sheet for this shift; the answer is the key SSC colour-marked on that sheet. Solution last reviewed 13 September 2026.

Where this paper and its answers come from

SSC does not publish CGL question papers or a public answer key. From 2025 the candidate response sheet prints every question and option as a picture, so the text here is transcribed from those pictures for this shift and checked against them independently; where the layout carries meaning, the picture is shown as well. Each answer is the key SSC colour-marked on that sheet. SSC released a tentative key for challenge and later its final one, both only to candidates behind a login, and the sheet does not say which of the two it carries. The solutions are written by UnlockIAS.

Which mirror type is used in solar furnaces to focus sunlight?

(a)Convex
(b)Plane
(c)Concave
(d)Biconvex

Answer

Why

Correct — C. To focus sunlight, a mirror must make parallel rays meet at a point, and the concave mirror is the type that converges them.

A solar furnace concentrates sunlight with parabolic mirrors, which are concave, often fed by flat heliostats. At the Odeillo furnace in France, the focus can reach about 3,500 °C → option (c).

Why the others are wrong

(a)
Convex — A convex mirror makes reflected rays spread apart, so they never meet. Its wide field of view suits a vehicle's rear-view mirror, not a furnace.
(b)
Plane — A plane mirror redirects light without converging it. Odeillo uses flat mirrors to steer sunlight onto its large curved mirror, but the focusing is done by the curved one.
(d)
Biconvex — Biconvex describes a lens, not a mirror. A convex lens does converge light, by refraction, but the stem asks for a mirror type.

Concept

A concave mirror curves inward and reflects parallel rays so they converge at its principal focus. A convex mirror curves outward and makes them diverge. A plane mirror does neither.

The Odeillo solar furnace in the French Pyrenees, operating since 1969, uses an array of plane mirrors that reflect sunlight onto a larger curved mirror, which concentrates it.

Heliostats are flat, so plane mirrors are part of a solar furnace. The stem asks what focuses the light, and that is the concave mirror.

Key facts

A concave mirror converges parallel rays to its principal focus, and a convex mirror diverges them.
Solar furnaces concentrate sunlight with parabolic mirrors, often fed by flat heliostats.
The Odeillo solar furnace in France began operating in 1969, and its focal temperature can reach about 3,500 °C.

Study next

Uses of concave and convex mirrors: headlights, shaving mirrors, rear-view mirrorsImage formation by a concave mirror at different object positionsConvex and concave lenses, and lens power in dioptres

Common traps

Choosing plane because solar furnaces use flat heliostats — they steer the light, and the concave mirror focuses it.
Choosing biconvex because it converges light — it is a lens, not a mirror.

Optics also appears at 13 Sep 2024, 12:30, GA Q.2 (the image of an object at the focus of a convex lens) and 11 Sep 2024, 12:30, GA Q.13 (the dioptre as the unit of a lens's power).

Related PYQs

No directly related past PYQ was found.

Topic: Scientific Research — all 220 SSC CGL questions on it, every solved shift.