It is often as a result of the eyeball being too short for the lens.There is a limit to how short the ciliary muscles can get the focal length to be.This equates to the maximum power of the lens, when the object is at the near point, typically 64 dioptres.
SCIENCE,TECHNOLOGY & SOCIETY Uses of lenses
Spectacles or contact lenses can be used to compensate for short- or long-sightedness. A diverging lens is used to compensate for short-sightedness. A converging lens is used to compensate for long-sightedness.
Short sight is caused when the eye bends light rays too sharply to focus on distant objects. It can be corrected with a diverging lens.
Long sight is caused when the eye cannot bend light rays sharply enough to focus on nearby objects. It can be corrected with a converging lens.
(a) Short sight
(b) Long sight Fig 13.32: Lenses can be used to correct (a) short sight and (b) long sight
SAMPLE PROBLEM 13G
An eye contains a lens system and a retina, which is 2.0 cm from the lens system.The lens system consists of the cornea, which acts as a fixed lens of power 38 m1, and a variable internal lens just behind the cornea, as illustrated in Fig 13.33.
Cornea Lens
Retina
Fig 13.33
The maximum power of the eye is 64 m1. Calculate: (i) how near an object can be placed in front of the eye and still be in focus (ii) the maximum power of the internal lens.
