Two thin convex lenses, each of focal length 20 cm, are placed coaxially 10 cm apart in air. The equivalent focal length of the combination is
A{'text': '13.3 cm', 'label': 'A'}
B{'text': '10 cm', 'label': 'B'}
C{'text': '20 cm', 'label': 'C'}
D{'text': '40 cm', 'label': 'D'}
Answer & Solution
Correct answer: A. {'text': '13.3 cm', 'label': 'A'}
1. For two thin lenses separated by distance d: 1/F = 1/f₁ + 1/f₂ − d/(f₁ f₂).
2. 1/F = 1/20 + 1/20 − 10/(20·20) = 0.05 + 0.05 − 0.025 = 0.075.
3. F = 1 / 0.075 ≈ 13.33 cm.
_Source: NCERT Class 12 Physics Part II, Ch 9 §9.5.4 (general two-lens formula)_
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