The sense organ for visual sensation is the eye.  Before we can see something in the external environment, a pattern of light from the object must first enter the eye through the cornea.  The right behind the cornea is a liquid called Aqueous Humor.  The amount of light entering the eye is regulated by the pupil which is an opening in the iris.  The iris is the part of the eye that gives its color passing through the lens.  This lens can be changed in curvature by the ciliary’s muscles in order to focus the light rays into a sensitive surface in the retina, however, the light rays must first pass through another liquid known as the vitrous humor.  As soon as the light rays reach the retina, chemical reactions are set up in the retinal receptors converting the light into more impulse.  These impulses from the retina are then carried to the brain by the optic nerve.

The retina is composed of two different kinds of photosensitive cells:

1.      Cones are receptors for colors.  Without the cones, this would be a colorless would to us.  Cone’s functions primarily on daylight.

2.      The Rods of the retina are not sensitive to colors.  Rods function primarily in dim light. 

The point where there are neither cones nor rods are called blind spot.

1. Nearsightedness or Myopia – the inability to see far objects. It is due to too long eyeball.
2. Farsightedness or hyperopia – the inability to see near objects. It is due to too short eyeball.
3. Astigmatism – due to imperfect curventure if the cornea
4. Cross-eyed – the convergence of the two eyes due to muscular weakness.
5. Total-blindness – due to the traumatic damage to the optic nerve.
6. Color-blindness – due to cones which are missing or malfunctioning (examples: Red – Green color blindness which is common and Blue – yellow color blindness which is rare).
7. Walleyed – excessive divergence of the two eyes.
8. Presbyopia or old sightedness – a condition where the lenses harden so that cannot change its shape.
9. Night blindness – defect where there is an inability to see well at night due to lack of Vitamin A.