alleryAn equiconvex thin lens has both surfaces of curvature 20 cm and refractive index 1.5. Its focal length is:
A10 cm
B15 cm
C30 cm
D20 cm
Answer & Solution
Correct answer: D. 20 cm
1. Lens-maker's formula: 1/f = (n - 1) * (1/R1 - 1/R2).
2. For equiconvex lens, R1 = +20 cm and R2 = -20 cm.
3. So 1/R1 - 1/R2 = 1/20 - (-1/20) = 2/20 = 1/10.
4. 1/f = (1.5 - 1) * 1/10 = 0.5 / 10 = 1/20.
5. f = 20 cm.
_Source: Selina Concise Physics ICSE Class 10, Ch 5 'Refraction through a Lens' (aplustopper.com extract)_
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