HUMAN EYE (STRUCTURE, IMAGE FORMATION AND DIFFERENCE BETWEEN RODS AND CONES)

Sight:

Eyes are the most superb of the sense organs as they make us mindful of different things around us, close-by and far away. When we see a tree, we don't need to touch it or trip its limbs so as to figure out what it is. With normal vision, we can reach out as far as our eyes can see and thus are aware of events around us. The eyes are organs of sight permitting us to acknowledge shape, color, and movement of items and persons around us.

Structure of Eye:

So as to understand how the eyes make us to see things we get to know how an eye is built. It is a little organ, sphere-shaped with the exception in the front area (the transparent area) where it bulges out a bit. In the normal individual, the eye is not as much of one inch (around 2.4cm) in diameter. Each eye is situated in a bony socket called orbit. The orbit is shaped partly by the bones that enclose brain and somewhat by the bones of the upper face. The socket contains muscles for the movement of the eye ball, the nerves for the connection to the brain and blood vessels that supply blood to the eye balls. Before each eye, there are eye-lids; an upper and a lower, which works together in order to shut out light. As covers (eye-lids) separate or when the eyes open, the front portion of the eye ball is exposed and light enters. Picture is formed by the light entering the eye through the transparent anterior portion.

Parts of Eye:

An eye has the following parts:

• Cornea               
• Iris                
• Lens             
• Sclerotic layer    
• Choroid layer        
• Retina
• Blind spot        
• Aqueous chamber and aqueous humour
• Vitreous chamber and vitreous humour

The sclera (outer layer) of the eye ball is white in color and is thick and firm. It is transparent in front and bulges out marginally to form the cornea (the transparent layer forming the front of the eye). Beneath sclera is another layer of the eye ball called choroid, which is pigmented and vascular. Behind the cornea the choroid curves inward to structure the iris having a round opening called pupil. Arranged behind the pupil is a biconvex lens, which experiences changes in convexity during the course of focusing. Nature has made suspensory ligaments and muscles to avoid dislocation of the lens. Retina is the third and inner most layer of the eye. It has optic nerves which connect the eye ball to the brain. Retina is the sensitive layer as it has sensitive cells which help in the formation of image and differentiation of colours in the picture. Retina has two types of cells:

• Rods
• Cones

These cells help in the formation of the picture on the retina. Cones separate colours while the rods are responsive to white light. There is a blind spot which lies above the point of entrance of the optic nerve.

The iris (flat, colored, ring-shaped membrane behind the cornea of the eye, with an adjustable circular opening (pupil) in the center) separates the eye into an aqueous anterior chamber containing a fluid (aqueous humour) and a vast posterior chamber, the vitreous chamber containing transparent jelly like fluid (vitreous humour). The pupil (aperture in the iris), the lens at the rear of the iris, aqueous and vitreous humour all are included in the focusing of a sharp picture on the retina.

Six muscles at all are present in the orbit (where the eye is situated) connected to the external side of sclera layer that causes the movement of the eye. Tear organs are also present in the orbit which produces secretions (tears) that wash the eyes.

Image Formation:

Incoming light rays from object are bent by cornea and lens so that
they come together at a single point at Retina.  Formation of Image in
inverted. The Brain ultimately interprets the image as rightside-up

Amid focusing, light beams pass through cornea and lens. The light beams reflected from the object at a distance striking the cornea are just about parallel to one another. Lens focuses the light rays on a special spot called fovea centralis (yellow spot) for creating sharp picture.

For clear image, the lens changes its convexity to create enough refraction in addition to that delivered by cornea to get the picture into focus and form a vivid picture on the retina.

Difference Between Rods and Cons:

Rods	Cons
1. Rods have cylindrical outer segments.	1. Cones have conical outer segments.
2. Rods' response is slow and have rhodopsin photo-pigment.	2. Cones' response is fast and have cone opsin photo-pigment.
3. The absolute sensitivity for Rods is High.	3. The absolute sensitivity for Cones is Low.
4. Rate of Dark adaptation for Rods is Slow.	4. Rate of Dark adaptation for Cones is High.
5. There is saturation in Rods and number of Ribbons are 1-2.	5. There is no saturation in Cones and number of Ribbons are 20-40.