question_answer

A converging and a diverging lens of equal focal lengths are placed coaxially in contact. Find the power and focal length of the combination.                                              

Answer:

                 Let \[f\] and \[-f\] be the focal lengths of the converging and diverging lenses respectively. \[P={{P}_{1}}+{{P}_{2}}=\frac{1}{f}+\frac{1}{-f}=0.\] Focal length of the combination. \[f=\frac{1}{P}=\frac{1}{0}=\infty \]