THE HUMAN EYE Gives the sense of sight. Allows us to learn more Sense organ Gives the sense of sight. Allows us to learn more about the surrounding world.

Topics covered Eye Structure Functions Image formation Eye Defects Correction of the Defects

External Appearance of the Eye

Functions Eye Lids: Cover and protect the eye by closing. Regular blinking distributes fluid over the eye. Prevent the eye from drying. Pupil: Determines the amount of light entering the eye. Sclera: Tough non elastic fibrous coat round the eye ball. Iris: Contracts or dilates to alter the pupil’s size.

The Structure of the Eye

Functions Cornea: Causes light to refract at the surface of the eye. Lens: It’s a converging lens. Curvature is controlled by ciliary muscles and suspensory ligaments. Causes light to undergo further refraction. Conjunctiva: Transparent membrane. Covers the sclera and lines the insides of the eyelids.

Choroid: Layer rich in blood vessels. Supply the eye tissues with oxygen and nutrients. Retina: Projection screen at the back where the images are focused. Once focused, vision is good. Optical nerve: Carries the signal from the retina to the brain. Cause of the blind spot in eyes.

Vitreous: Transparent gel. Fills the main cavity of the eye. Macula: A depression in the retina cones (photoreceptor cells) are concentrated and vision is most acute.

Image formation

Process of sight When you look at an object, The light rays are bent, refracted and focused by the cornea, lens, and vitreous. The lens makes the rays come to a sharp focus on the retina. The resulting image on the retina is inverted, diminished and real. The light rays are converted to electrical impulses which are then transmitted through the optic nerve, to the brain, where the image is translated and perceived in an upright position!

Short sightedness This person can’t see Causes: far things clearly because light rays meet in front of the retina. Causes: eye ball is too long. eye lens is too strong. cornea is too thick.

Correction of Short sightedness The person should put on glasses with concave (diverging) lenses so as to focus far objects.

Long Sightedness This person can’t see near things clearly because light rays meet behind of the retina Causes: eye ball is too short. eye lens is too weak. cornea is too thin.

Correction of Long sightedness The person should put on glasses with convex (converging) lenses so as to focus near objects.