The same size images are formed by a convex lens when the object is placed at or at from the lens. The focal length of a convex lens is ______ .
Step 1: Given Data
Images formed by a convex lens when the object is placed at is
Images formed by a convex lens when the object is placed at is
Object distance in a convex lens is taken as negative due to the sign convention.
Step 2: Formulas Used
= magnification
is the focal length of the lens.
is the distance between the object and the optical center.
is the distance of the image formed from the optical center of the lens.
Step 3: Calculating the focal length
It is already given in the question that the magnification is the same in both cases.
Object distance in a convex lens is taken as negative due to the sign convention and a convex lens has a positive focal length, hence magnification at is taken as negative.
The focal length of the convex lens is .