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What structures of the eye makes vision possible?

The eye is about an inch in diameter and for a person to be able to see an object light must reflect off that object. The center of the eye is covered by a transparent thick liquid, vitreous  which determines the shape of the eye. The front clear portion of the eye is known as the cornea and the amount that enters the eye is controlled by the iris which dictates the size of the pupil. Behind the iris, a crystalline structure is found that assists with the focus of light into the retina is known as lens. The retina lines the back of the eye and it is made up of photoreceptors known as rods and cones and it is in this area that light is converted into electical impulses. Rods sense brightness and cones sense color. The cranial nerve that transmis electrical impulses from the retina to the brain is known as the optic neve. The eyes also has glands in the eyes that contains a clear liquid known as tears. 

What conditions affect vision?

A normal person has 20/20 vision but with age and other factors a person's vision begins to deteriorate.

• Astigmatism affects the ability of the retina to focus on light causing blurry vision.
• Diabetic retinopathy is very common in people with diabetes and this is due to high levels of glucose that damages the blood vessels in the eye causing the blood vessels to start leaking, possibly lead to complete vision loss.
• Glaucoma is the loss of vision over a period of time caused by an increase pressure inside of the eye.
• Hyperopia is known as farsightedness and it is the inability to see objects clearly that are found at closed proximities.
• Myopia is known as nearsightedness and it is the inability to see clearly at a distance because the retina is not focused on light.
• Optic neuritis is when the optic nerve is inflamed 

How is vision loss diagnosed?

The initial exam given to every person regardless the age is known as the visual acuity test where a person stands or sits in front of an eye chart and is asked to read the numbers and letters down each line. Retinoscopy is a way to detect how light reflects from the eye and which lens prescription are necessary to improve vision. A hird visual exam is known as an external exam and pupillary reactions test which examines how both pupils react when objects and light are at close distance.