Table showing focal length of three concave mirrors A, B and C and the respective object distances.
Study the data given below showing the focal length of three concave mirrors A, B and C and the respective distances of objects placed in front of the mirrors :
Generated by claude-sonnet-4-6 · 2026-06-15 06:51 · grounding stimulus
Model Answer
(i) Case 1 (Mirror A) will form a diminished image.
Justification: Object distance (45 cm) > 2f (40 cm), so the object is beyond C. A concave mirror forms a diminished image when the object is placed beyond the centre of curvature.
(ii) In Case 2, f = 15 cm, object distance = 30 cm = 2f (at centre of curvature). Properties of image:
- Real and inverted
- Same size as the object (at C)
(iii) For Mirror C: f = 30 cm, object distance = 20 cm < f. The object is placed between the pole and focus.
Nature and size: Virtual, erect, and magnified image is formed behind the mirror.
Ray Diagram:
(Diagram showing object between P and F of concave mirror, with two rays — one parallel to principal axis reflecting through F, and one directed towards F reflecting parallel — diverging after reflection, with virtual image formed behind mirror, larger than object.)
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Explanation
- For (i), compare object distance with 2f: if u > 2f → diminished image (beyond C rule).
- For (ii), object at C (u = 2f) always gives real, inverted, same-sized image — a standard result to memorise.
- For (iii), object between P and F is the only case where a concave mirror gives a virtual, erect, magnified image. CBSE expects the ray diagram with at least two rays clearly showing the virtual image behind the mirror. Label P, F, C on the diagram.