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

Options

A. `(770)/(36)` cm
B. `(77)/(36)` cm
C. `(36)/(770)` cm
D. `(360)/(77)` cm

Answer

A

Explanation

Here, distance between object and screen D= `90` cm
Distance between two locations of convex lens
d= `20` cm, f= ?
As, `f=(D^(2)-d^(2))/(4D)=(90^(2)-20^(2))/(4xx90)=(110xx70)/(360)=(770)/(36)` cm