Question

why do our eye lens act as low converging power lens when they are thin (extended by ciliary muscles) .?

Answer 1

When the eye lens is extended to make it thin, the curvature of the lens goes down. Now we know that the focal length of the lens depends on the radius of curvature. Focal length of the spherical lens is proportional to the radius of curvature. Thus when the eye lens become thin, focal length increases and hence become less converging.