Human Eye and Colourful World

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• Questions
• Summary

Hypermetropia (Far sightedness)

• The affected person can see far objects clearly but cannot see nearby objects clearly.
• The near point of the eye moves away.
• An image is formed behind the retina.

Causes of Hypermetropia

• The focal length of the eye lens becomes too long.
• Eyeball becomes too small.

Correction
Use of convex lens of suitable power can correct the defect.

Hypermetropia (Far sightedness)

• The affected person can see far objects clearly but cannot see nearby objects clearly.
• The near point of the eye moves away.
• An image is formed behind the retina.

Causes of Hypermetropia

• The focal length of the eye lens becomes too long.
• Eyeball becomes too small.

Correction
Use of convex lens of suitable power can correct the defect.

Myopia (Nearsightedness)

• A myopic person can see nearby objects clearly but cannot see distant objects clearly.
• An image is formed in front of the retina.

Causes of Myopia

• The excessive curvature of the eye lens
• Elongation of eyeball

Correction
Use of concave lens of appropriate power.

(a) In a myopic eye, image of distant object is formed in front of the retina (and not on the retina)

(b) The far point (F) of a myopic eye is less than infinity

(c) Correction of myopia. The concave lens placed in front of the eye forms a virtual image of the distant object at far point (F) of the myopic eye.

Presbyopia (Old age Hypermetropia)

• It is the defect of vision due to which an old person cannot see the nearby objects clearly due to loss of power of accommodation of the eye.
• The near point of the old person having presbyopia gradually recedes and becomes much more than 25 cm away.

Causes

• Gradual weakening of ciliary muscles.
• Diminishing flexibility of eye lens.

Correction

• Use of convex lens of suitable power.
• Sometimes a person may suffer from both myopia and hypermetropia.
• Such people require bifocal lens for correction.