Question

An object placed $10cm$ in front of a lens has an image $20cm$ behind the lens. What is the power of the lens (In dioptre)?

• A. $+0.15$
• B. $-0.15$
• C. $-1.5$
• D. $+1.5$

Answer 1

Answer: (A)

$+0.15$

$\begin{array}{c}u=-10cm,v=20cm\\ \text{Lens formula}\\ \therefore \frac{1}{v}-\frac{1}{4}=\frac{1}{f}\\ \therefore \frac{1}{20}-\frac{1}{(-10)}=\frac{1}{f}\\ \Rightarrow \frac{1+2}{20}=\frac{1}{f}\\ \Rightarrow \frac{1}{f}=\frac{3}{20}cm\\ \text{Now}p=\frac{3}{20}\times 200=15D\\ \text{Hence,}+15D\text{is correct answer.}\end{array}$