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  4. Two thin lenses of focal length 20 cm and 25 cm are in contact. The effective power of the combination is

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Q. Two thin lenses of focal length $20 \, cm$ and $25 \, cm$ are in contact. The effective power of the combination is

NTA AbhyasNTA Abhyas 2020Ray Optics and Optical Instruments

Solution:

According to the question
$P_{1}=\frac{100}{20}=5 \, D, \, P_{2}=\frac{100}{25}=4D$
Effective power $P=P_{1}+P_{2}$
$=5+4=9D$