alleryA convex lens of focal length 20 cm is placed in contact with a concave lens of focal length 25 cm. The power of the combination (in dioptres) is
A+9.0 D
B+1.0 D
C+0.05 D
D−1.0 D
Answer & Solution
Correct answer: B. +1.0 D
1. Combined power P = P₁ + P₂.
2. P₁ = 1 / 0.20 = +5 D; P₂ = −1 / 0.25 = −4 D.
3. P = +5 − 4 = +1 D.
_Source: syllabus Physics Part II, Ch 9 §9.5.4_
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