When an object is placed at the centre of curvature, 2F1, of a convex lens, its image is formed at the centre of curvature, 2F2, on the other side of the lens. The image formed is inverted and of the same size as the object, as shown in the given figure.
It is given that the image of the needle is formed at a distance of 50 cm from the convex
lens. Hence, the needle is placed in front of the lens at a distance of 50 cm.
Object distance, u = −50 cm
Image distance, v = 50 cm
Focal length = f
According to the lens formula,