Q. How can the visual deficiencies known as myopia and hypermetropia be optically explained?
Myopia is the visual condition in which the images are formed previous to (in front of) the retina. Hypermetropia is the visual condition in which the point of image formation is further than (behind) the retina. Actually myopia is due to an increase in the distance between the crystalline lens and the retina, generally caused by a slight flattening of the eyeball and in hypermetropia the retina is too close to the crystalline lens due to slight shortening of the eyeball.
In myopia the far point and the near point of vision come closer (the refractivity of the crystalline lens that corresponds to the maximum distension capacity of the ciliary muscles isn't enough to provide visual accommodation). In hypermetropia the ciliary muscles aren't able to contract more to compensate the inadequate position of the retina that is the near point becomes more distant.