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You must have seen some people holding books very close to their eyes. This is because they suffering from myopia (short sightedness). A myopic person cannot see distant objects clearly because the far point of his eye is less than infinity.
Myopia is an eye defect due to which the eye is not able to see distant
objects clearly.
This occurs when the light rays entering the eye converge in front of the retina
in the vitreous body.
Here you can see the formation of image in a normal eye and in a myopic eye.
 
In the case of a normal eye the rays of light from the object fall on the eye
and converge on the retina but in the case of a myopic eye the light rays are
focused in front of the retina.
Myopia is the defect of the eye due to which the eye is not able to see the distant objects clearly.
Myopia is due to:
- the elongation of the eye ball, that is, the distance
between the retina and eye lens is increased
- decrease in focal
length of the eye lens
A myopic eye forms the image of a far off object in front of the retina because of the increase in converging power of the eye lens. Therefore myopia can be rectified by using a suitable divergent or concave lens.
The ray diagram shows how a concave lens helps in focusing the light on the
retina. 
This defect is commonly known as nearsightedness.
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