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

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

WBJEEWBJEE 2011Ray Optics and Optical Instruments

Solution:

We have the effective power given by
$ P_{eff}=P_1+P_2$
Also we know, $ P = \frac{1}{f}$
Hence, $ P_{eff}= \frac{1}{f_1}+\frac{1}{f_2}$
$ = \frac{1}{0.20} + \frac{1}{0.25} = 5+ 4 = 9D $