Question

A screen is placed 90 cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20 cm The focal length of the lens is

• A. $\frac{770}{36}\, cm$
• B. $\frac{77}{36}\, cm$
• C. $\frac{36}{770}\, cm$
• D. $\frac{360}{77} \,cm$

Answer 1

Answer: (A)

Here, distance between object and screen $D=90 \,cm$
Distance between two locations of convex lens
$d=20 \,cm , f=$ ?
As, $f=\frac{D^{2}-d^{2}}{4 D}=\frac{90^{2}-20^{2}}{4 \times 90}$
$=\frac{110 \times 70}{360}=\frac{770}{36} \,cm$