Final answer:
Hyperopia and presbyopia can cause an inability to focus on close objects. Hyperopia is when the eye is unable to converge the rays from a close object on the retina, resulting in blurry close vision. Presbyopia is a vision problem associated with aging where the eye gradually loses its ability to focus on close objects.
Step-by-step explanation:
The inability to focus on close objects can be caused by hyperopia or presbyopia. Hyperopia, also known as farsightedness, occurs when the eye is unable to converge the rays from a close object by the time they strike the retina, resulting in blurry close vision. This can be caused by insufficient power in the lens or by the eye being too short. Presbyopia, on the other hand, is a vision problem associated with aging in which the eye gradually loses its ability to focus on close objects. It is believed to be caused by the lens becoming less elastic and the muscles controlling the lens losing power as people grow older.