In order to see objects at different distances clearly, a person's eyes must be able to adjust the focal length of their lenses. if person can't achieve the full range of focal lengths, they will require corrective lenses to have "normal" vision. the corrective lenses in glasses usually sit about 2cm from a person's eyes. someone who is farsighted cannot see clear images of objects close up. one such person is unable to see clear images of objects closer than 42 cm from their eyes, whereas normal human vision can see clear images of objects as close as 25cm from their eyes. if this farsighted person get glasses to correct their vision, what should the focal length of those glasses be?