Question

A point object is kept at the first focus of a convex lens. If the lens starts moving toward right with a constant velocity, the image will 

(A) Always move toward right 

(B) Always move toward left 

(C) First move toward right and then toward left. 

(D) First move toward left and then toward right

Answer 1

The correct option is (D) First move toward left and then toward right.

Explanation:

Let as work in the frame of reference attached to the lens. Lens formula:

1/v - 1/u = 1/f

Differentiating w.r.t. time

Initially when u = f, v → ∞ so speed image is very large and finally when u ,v f →∞ → and the speed of image is very low (nearly zero). With respect to lens, as object moves left, the image also moves left.

Speed of image with respect to object is