alleryTwo thin lenses of focal lengths +20 cm and +25 cm are placed in contact. Power of combination:
A+9 D
B+5 D
C+10 D
D+1 D
Answer & Solution
Correct answer: A. +9 D
1. Each power: P = 1 / f.
2. P1 = 1 / 0.20 = 5 D and P2 = 1 / 0.25 = 4 D.
3. Combination power = P1 + P2.
4. P = 5 + 4 = +9 D.
5. So the combination is +9 dioptres.
_Source: Selina Concise Physics ICSE Class 10, Ch 5 'Refraction through a Lens' (aplustopper.com extract)_
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