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How do we see things? / Principle of image formation

The retina, the innermost structure of the eye, behaves like a projection screen which receives the inverse images and the cerebral centre of the brain redresses (form an erect) the image.

Types of refractive errors:

• Emmetropia.
• Hyperopia or Farsightedness.
• Myopia or Nearsightedness.
• Astigmatism.
• Presbyopia

Emmetropia(Normal vision):

• Emmetropia.
• A condition where no refractive error is presents.

Hyperopia or Farsightesness:

• Eyeball is too short.
• Clear distance vision but suffer with eyestrain for near work.
• Correction by means of spherical plus lenses.

Myopia or Nearsightedness:

• Eyeball is too long.
• Clear near vision and blurred distant vision, people generally squeeze their eyes to see the distant object
• Correction by means of spherical minus lenses.

Astigmatism:

• A point object will not have a point image.
• Blurred vision for distance and near.
• Generally have eyestrain and headache.

Presbyopia:

• Lens begin to lost its elasticity/accommodating capacity.
• Difficulty in focusing near object and clear distant vision.
• Symptoms begins at the age of forty.
• Corrected by means of bifocal/progressive lenses for near and intermediate vision.