Question

For more than two thin lenses kept in contact, the equivalent focal length \[f\] satisfies:

Options

• \[\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}+\frac{1}{f_3}+\ldots\]

• \[f=f_1+f_2+f_3+\ldots\]

• \[\frac{1}{f}=\frac{1}{f_1+f_2+f_3+\ldots}\]

• \[f=f_1 f_2 f_3 \ldots\]

Answer 1

The power-addition principle extends to any number of thin lenses in contact: the reciprocal of the equivalent focal length equals the sum of the reciprocals of the individual focal lengths, i.e. \[\frac{1}{f}=\frac{1}{f_1}+\frac{1}{f_2}+\frac{1}{f_3}+\ldots\].