Science — CBSE Class 10 board question

(i) Principal Focus of a Diverging (Convex) Mirror:
The point on the principal axis from which incident rays, parallel to the principal axis, appear to diverge after reflection from a convex mirror is called its principal focus (F). It is a virtual focus, located behind the mirror.

(ii) Focal Length:
The distance between the pole (P) of the convex mirror and its principal focus (F) is called the focal length. It is positive for a convex mirror.

Ray Diagram:

```
Incident ray (parallel to principal axis)
──────────────────►
\ Reflected ray (diverging)
\─────────────────►
───────────────F────────P ( ( ( ( convex mirror
*
(Virtual focus F, behind mirror)
```

(Label: P = Pole, F = Principal Focus, C = Centre of Curvature — all behind the mirror)

Source: Chapter 9, Section 9.2.2

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• Examiners expect both definitions clearly stated (1 mark each) and a labelled diagram (1 mark).
• Key point: the principal focus of a convex mirror is virtual — reflected rays only appear to diverge from F; they don't actually meet there.
• In the diagram, show the incident ray parallel to the principal axis, the reflected ray going outward, and a dotted line extended behind the mirror meeting at F. Label P, F, and C.
• Focal length for a convex mirror is taken as positive under New Cartesian Sign Convention.