Question

Two lens of focal lengths $-20cm$ and $+10cm$ are put in combination. Find the power of the combination.

• A. $-1D$
• B. $-2D$
• C. $+5D$
• D. $+2D$

Answer 1

The correct option is C $+5D$

Power of the concave lens $=\frac{1}{f}=\frac{100}{-20}D=-5D$

Power of the convex lens $=\frac{1}{f}=\frac{100}{10}D=+10D$

Thus, the power of the combination $=P_1+P_2=+10D-5D=+5D$