A luminous object is separated from a screen by distance d. A convex lends is placed between the object and the screeen such that it forms a distinct image on the screen. The maximum possible focal length of this convex lens is.

Options

A. 4d
B. 2d
C. `(d)/(2)`
D. `(d)/(4)`

Answer

D

Explanation

Focal length of a convex lens by displacement method
`f=(a^(2)-b^(2))/(4a)`
where a = distance between the image and object and b = distance between two positions of lens
Fact The distance between the two points should be greater than four time the focal length of the convex lens, i.e. `d gt 4f`
So, the maximum possible focal length of this convex lens is `(d)/(4)`.