Sample 1Medium
A ray diagram shows a concave mirror of focal length 15 cm with an object placed on its principal axis 20 cm in front of the pole, between the focus F and the centre of curvature C. Where is the image formed and what is its nature?
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The answer is B.
1/v + 1/u = 1/f with u = −20 cm and f = −15 cm gives 1/v = −1/60, so v = −60 cm (in front). m = −v/u = −3, so the image is real, inverted and magnified three times, beyond C.
- A.
A virtual, erect image forms only when the object is between P and F.
- C.
8.6 cm comes from adding 1/15 and 1/20 instead of subtracting.
- D.
A same-size image forms only when the object is at C (30 cm).
NCERT: Class 12 Physics, Chapter 9