The near point of a hypermetropic eye is 1m. What is the power of the lens required to correct this defect ? Assume that the near point of the normal eye is 25 cm.

The eye defect called Hypermetropia is corrected by using a convex lens.

We will first calculate the focal length of the convex lens required in this case. For hypermetropic eye can see the nearby object kept at 25 cm (at near point if normal eye) clearly if the image of this object is formed at its own near point which is 1 meter here. So, in this case :

Object distance, u = -25 cm (Normal near point)

Image distance, v = -1 m (Near point of this defective eye) = -100 cm

Focal length, f= ? (To be calculated)

Putting these values in the lens formula,

We get :

Or,

Thus, the power of convex lens required is +0.3 diopters.

Myopia and Hypermetropia are the two most common defects of vision. We will now study another defect of vision which occurs in old age. It is called presbyopia.