Explain with the help of labelled ray-diagram, the defect of vision called hypermetropia, and how it is corrected by a lens.

Hypermetropia, also known as long-sightedness or far-sightedness, is a defect of vision in which a person can't see the nearby object clearly (appears blurred), though can see the distant objects clearly. The near point of a hypermetropic eye is more than 25 cm away.

This defect occurs either due to the low converging power of the eye lens, (because of its large focal length). Or, due to the eye-ball being too short, which causes light to focus beyond the retina, instead of directly on the retina. As a result, the person cannot see these objects clearly. 

It is corrected by using spectacles containing convex lenses, which should be of such a focal length (or power) that it forms a virtual image of the object (lying at the normal near point N of 25 cm) at the near point 'N' of the hypermetropic eye. Due to which the eye lens then easily focuses the light rays from the near point and forms clear images of the objects on the retina.

The following ray diagram explains the defect of vision called hypermetropia, and how it is corrected by a lens.