Human visual system consists of the Eyes, several parts of
the brain, and the pathways connecting them.
The Eye
Eye consists of two systems:
1. Image forming system
2. Image transduction system
In this article, we will just discuss image forming system.
Image Forming System
This system consists of following structures:
Cornea
Cornea is the front surface of eye. Light enters the eye
through cornea. It is a transparent protective tissue covering front of the
eyeball. As the cornea is deeply curved, it bends the rays of incoming light
and helps to focus them.
Pupil
Pupil is a circular opening which contracts or expands
automatically, in response to incoming amount of light. Therefore, the size of
pupil opening depends on the amount of light in the surroundings.
Iris
This is the colored part of the eye that ranges from light
blue to dark brown.
Sclera
The sclera is commonly known as "the white of the
eye." It is the tough, opaque tissue that serves as the eye's
protective outer coat.
Lens
Located behind the pupil, focuses the light by changing its
own thickness, through a process called Accommodation (the ability of the lens
to vary its shape in order to focus incoming images on the retina)
Kind of accommodation depends on the location of the object
Distant
objects → relatively flat lens
Close objects
→ Thick, rounder lens
Having traveled through pupil and lens, light waves reach
its ultimate destination in the eye.
Retina
The retina is the series of the nerve cells at the back of
the surface of the eye. Retina converts
the electromagnetic energy of into useful information for the brain. There are
two types of the light sensitive receptor cells in retina:
Rods
Rods are long and cylindrical in shape. Rods are more
sensitive to light and work in dim light. They respond to difference in
brightness and are most important for seeing in dim light. They are sensitive
to light waves in 500nm range, or to blue and green light. The images produced
by rods are in black, white and gray.
Cones
Cones are short, thick and cone shaped. Cones respond to
color or hue. They require more light. They are responsible for sharp focus and
color perception, particularly in bright light. Cones are more sensitive to
wavelength of 550nm range but they respond to all colors. They are tightly
packed together in the middle of retina and this is the area around fovea.
Fovea
Fovea is the part of retina where the concentration of cones
is greatest. Images are seen more sharply when they fall on the fovea. It helps
in focusing on something of particular interest.
Optic Nerve
Optic nerve is the fiber composed of many neurons and is
located at the back of the eyeball that carries neural impulses (visual
information) from the eye to the brain.
Blind Spot
Blind Spot is the point at which the optic nerve leaves the
eye, creating a “blind spot”. This point has no receptor cells.
Optic Chiasm
A point between and
behind the eyes at which nerve impulses from the optic nerves are reversed and
“righted” in the brain.