Hold a concave mirror in your hand and direct its reflecting surface towards the sun. Direct the light reflected by the mirror on to a white card-board held close to the mirror. Move the card-board back and forth gradually until you find a bright, sharp spot of light on the board. This spot of light is the image of the sun on the sheet of paper; which is also termed as "Principal Focus" of the concave mirror.
Generated by claude-sonnet-4-6 · 2026-06-15 06:52 · grounding stimulus
Model Answer
(a) Two applications of a concave mirror:
- Used as a shaving/makeup mirror (gives magnified, erect image).
- Used as a reflector in torches, headlights, and solar furnaces.
(b) Focal length (f) = 15 cm
Radius of curvature R = 2f = 2 × 15 = 30 cm
(c) Ray diagram — object placed between pole (P) and focus (F):
```
/
Object / (Reflected rays diverge)
↑ / ←──────────────────────────
| ( Mirror Virtual, erect,
P F magnified image
behind the mirror
```
Key features: Incident ray parallel to principal axis reflects through F; incident ray through C reflects back on itself. Reflected rays diverge — when extended behind the mirror, they meet to form a virtual, erect, and magnified image behind the mirror.
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Explanation
- (a) Any two standard applications suffice for 1 mark — examiners accept torch/headlight reflector, solar furnace, shaving mirror, ENT doctor's mirror, etc.
- (b) The formula R = 2f is the key relationship. Always show the working step for a calculation question.
- (c) For 2 marks, examiners look for: correct position of object (between P and F), at least two correct rays drawn, reflected rays shown diverging, and image correctly labelled as virtual, erect, and magnified behind the mirror. Since a proper diagram cannot be drawn in text, draw it neatly on your answer sheet with labelled points P, F, C, object, and image.