A student took three concave mirrors of different focal lengths and performed the experiment to see the image formation by placing an object at different distances with these mirrors as shown in the following table.
Answer the sub-parts based on the experiment with three concave mirrors as given in the table.
Generated by claude-sonnet-4-6 · 2026-06-15 06:52 · grounding stimulus
Model Answer
(a) In Case I, object-distance = 45 cm, focal length = 20 cm → radius of curvature = 40 cm. Object is beyond C (45 > 40).
Properties of image:
- Real and inverted
- Diminished (smaller than object)
(b) Case II — object-distance = 30 cm, focal length = 15 cm → radius of curvature = 30 cm. The object is placed at the centre of curvature (C). When an object is placed at C, a concave mirror forms a real, inverted image of the same size at C itself.
(c) Dentists use a concave mirror. It is used because when a tooth (object) is placed within the focal length of a concave mirror, it produces a magnified, erect, and virtual image, enabling the dentist to clearly examine the tooth.
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Explanation
- For (a): Compare object-distance with 2f (= radius of curvature). 45 > 40, so object is beyond C → image is real, inverted, and diminished.
- For (b): Image is same size only when object is placed at C (u = 2f). Case II: 2f = 30 cm = object-distance. This is the key condition to state.
- For (c): A standard fact — dentists/doctors use concave mirrors for magnified images. Always justify with the property (virtual, erect, magnified when object is inside focus).