The human eye

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This topic is for students studying the full biology course only

The structure and function of the eye

The eye is a complex organ composed of several parts, each playing a vital role in vision:

Sclera: The tough outer layer that protects the eye.
Cornea: The clear front part that allows light to enter.
Iris: The colored part that controls the size of the pupil.
Pupil: The black opening that lets light into the eye.
Lens: A transparent structure that focuses light onto the retina.
Retina: The light-sensitive layer at the back of the eye that converts light into electrical signals.
Optic Nerve: Carries these signals to the brain.
Ciliary Muscles and Suspensory Ligaments: Control the shape of the lens for focusing.

The iris reflex helps protect the retina from excessive light:

When exposed to bright light, the pupil contracts to reduce the amount of light entering the eye.
When exposed to dim light, the pupil dilates to increase the amount of light entering the eye.

The eye adjusts its focus by changing the shape of the lens:

When focusing on near objects, the lens becomes more curved.
When focusing on distant objects, the lens becomes less curved.

Correcting defects of the eye

People with long-sightedness (hyperopia) have difficulty focusing on near objects. This is often caused by the lens being too weak or the eyeball being too short.

People with short-sightedness (myopia) have difficulty focusing on distant objects. This is often caused by the lens being too strong or the eyeball being too long.

These defects can be correct with:

Glasses are the most common and simplest option for correcting vision problems.
Contact lenses are thin lenses that sit directly on the eye. They are a good option for people who prefer not to wear glasses.
Laser eye surgery is a procedure that uses a laser to reshape the cornea. It can be a good option for people with certain vision problems.
Replacement lens surgery is a procedure that involves removing the natural lens of the eye and replacing it with an artificial lens. This is often used to treat cataracts, a clouding of the natural lens.

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