Question

Two lenses of power $+\, 12 \,D$ and $-2\, D$ are combined together. What is their equivalent focal length ?

• A. 16.6 cm
• B. 10 cm
• C. 8.33 cm
• D. 12.5 cm

Answer 1

Answer: (B)

10 cm

Answer 2

Given: Power of first lens $\left(P_{1}\right)=+12\, D$ and power of second lens $\left(P_{2}\right)=-2\, D$. We know that power of the combination $(P)=P_{1}+P_{2}=12+(-2)=+10\, D$. $\therefore\,\,\,\,$ Equivalent focal length $(F)=\frac{1}{P}=\frac{1}{10} m =10 \,cm$