These two are the defect of the eye.
Hypermetropia : A person suffering from this defect can see distant objects clearly but cannot see nearby objects clearly. In this defect the near point lies farther away than 25 cm.
This defect is due to the following reasons:
- Either the hyperopic eyeball is too short or
- The ciliary muscle is unable to change the shape of the lens enough to properly focus the image i.e., the focal length of the eye lens increases. The condition can be corrected with a converging or convex lens.
Myopia: A person suffering from myopia or short sightedness can see nearby objects clearly but cannot see the far away objects clearly.
This defect occurs due to the following reasons:
- When the eyeball is longer than normal or
- When the maximum focal length of the lens is insufficient to produce a clearly formed image on the retina.
Myopia can be corrected with a diverging or concave lens.