A convex lens made of glass forms a sharp image on the screen for a particular position of an object with respect to the lens. A human eye lens is also a convex lens but it can form sharp images on the retina of eye for different positions of the objects. Explain, why?

A convex lens made of glass has a fixed focal length and hence it forms a sharp image on the screen for a particular position of an object. However, the focal length of human eye lens can be changed by the action of ciliary muscles. In other words, human eye lens has the ability to charge its focal length to form sharp images of objects at different positions. The process is known as accommodation of eye.