Question

The distance between an object and the screen is 100 cm. A lens produces an image on the screen when placed at either of the positions 40 cm apart. The power of the lens is

• A. 3D
• B. 5D
• C. 7D
• D. 9D

Answer 1

The correct option is B 5D

By using $f=\frac{D^2-x^2}{4D}\Rightarrow =\frac{{100}^2-{40}^2}{4\times 100}=21cm$
Hence power $P=\frac{100}{F\left(m\right)}=\frac{100}{21}\approx +5D$