Question

Two lenses one of focal length $20cm$. (convex lens) and another of focal length $-15cm$ (concave lens) are placed in contact. Identify the correct statement.

• A. The focal length of combination is $-0.6m$
• B. The focal length of combination is $\frac{0.6}{7}$ m
• C. The power of the combination is $-\frac{5}{3}$ D
• D. The power of the combination is $\frac{35}{3}$ D

Answer 1

Answer: (C)

The power of the combination is $-\frac{5}{3}$ D

Focal lengths $f_1=20cm=0.20m;f_2=-15cm=-0.15m$

Power of each lens$P_1=\frac{1}{f_1}=\frac{1}{0.20}=5D;P_2=\frac{1}{f_2}=\frac{1}{-0.15}=-6.67D$

The the power of the combination of lenses $=5+(-6.67)=-1.67D=\frac{-5}{3}D$