[Since the eye of mammals has a fixed image distance, in order to focus images of objects at various distances from the eye onto the retina, the lens of the eye has a variable focal length.] Find the change in the focal length of the eye when an object originally at 4 m is brought to 32 cm from the eye. [The retina is 2.5 cm from the lens of the eye.] Answer in units of mm. An explanation would be appreciated.