For a thin lens, the focal length is the distance from the optical centre to
A{'text': 'the second refracting surface', 'label': 'A'}
B{'text': 'twice the radius of curvature', 'label': 'B'}
C{'text': 'the centre of curvature of either surface', 'label': 'C'}
D{'text': 'the principal focus where parallel paraxial rays meet (or appear to diverge from)', 'label': 'D'}
Answer & Solution
Correct answer: D. {'text': 'the principal focus where parallel paraxial rays meet (or appear to diverge from)', 'label': 'D'}
1. Parallel paraxial rays striking a convex lens converge at F; for a concave lens, they appear to diverge from F.
2. The optical centre to F distance is the focal length f.
3. (A) and (D) confuse f with R, which are related (f = R/2 only for spherical mirrors of small aperture).
_Source: NCERT Class 12 Physics Part II, Ch 9 §9.5.2_
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