Explain myopia, hypermetropia, pressbiopia

Myopia

• Near-sightedness : A person with Myopia can see nearby objects clearly
• A person with myopia cannot see faraway objects clearly.
• The far point for the myopic eye is nearer than infinity
• Occurs due to
  • Excessive curvature of the eye lens
  • Elongation of eyeball
• The image of a distance object is formed in front of the retina and not on the retina
• Defected is corrected by using Concave lenses such that the lens will bring the image back on to the retina.

Hypermetropia

• Far sightedness : A person with Hypermetropia can see faraway objects clearly.
• A person with Hypermetropia cannot see nearby objects clearly.
• The near point of the eye is more than 25cm
  • This arises mostly during latter stages in life, as a result of the weakening of the ciliary muscles and/or the decreased flexibility of the lens.
  • Focal length of the eye lens is too long
  • Eyeball has become too small. 
• The image of a distance object is formed in behind the retina and not on the retina
• Defected is corrected by using Convex lenses such that the lens will bring the image back on to the retina.

Presbyopia

• The power of accommodation of the eye usually decreases with ageing. The ciliary muscles weaken and thereby the flexibility of the eye lens reduces.
• The near point moves away.
• Spectacles with convex lenses are recommended