THE EYE AND HOW IT WORKS
Your eyes are like cameras, but instead of film, the human eye focuses light onto a sensitive membrane called the retina. This is how the eye works:
The cornea is a transparent structure situated in the front of the eye that helps to focus incoming light. Behind the cornea is a pigmented membrane called the iris which has an adjustable circular opening called the pupil. The pupil expands and contracts depending on the amount of light entering the eye.
The space between the cornea and iris is filled with a transparent fluid called aqueous humor. Behind the pupil is situated a transparent crystalline structure called the lens. The lens is surrounded by muscles called the ciliary muscles which play an important role in vision. When these muscles are relaxed, they pull and flatten the lens, allowing the eye to see objects that are far away. In order to see near objects, the ciliary muscles have to contract causing the lens to thicken, thus enabling clear vision.
After passing through the lens, light travels through a jelly-like substance called the vitreous humor, before striking the sensitive layer of cells called the retina. The retina is the innermost of the three layers that make up the eye. The outermost layer is made of a tough protective tissue called the sclera. This is what gives the eyeball its white colour. The cornea is also part of the outer layer. The middle layer between the retina and the sclera is called the choroid. The choroid contains blood vessels that supply the retina with nutrients and oxygen.
There are millions of light sensitive cells embedded in the retina. They are mainly of two varieties: the rods and the cones. Rods are useful for vision in dim light while cones are used for colour vision. Cones are most densely packed in the centre of the retina called the fovea. The fovea is situated within the macula and is the most light sensitive portion of the retina. When light strikes these light sensitive cells, it is converted into electric signals that are then relayed to the brain via the optic nerve. The brain then translates these electric signals into the images we see.
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