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Two convex lenses of focal lengths 20 cm and 50 cm are placed in contact. The power of the combination is:
A+5 D
B+9 D
C+7 D
D+10 D
Answer & Solution
Correct answer: C. +7 D
1. Power of a lens P = 1 / f (in metres).
2. P1 = 1 / 0.20 = 5 D and P2 = 1 / 0.50 = 2 D.
3. For lenses in contact, the combined power is P = P1 + P2.
4. P = 5 + 2 = 7 D.
5. So the answer is +7 dioptres.
_Source: NCERT Class 10 Science Ch 11 'Human Eye and the Colourful World'_
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