The Science of Sight

The human eye is our window to the beauty of the world. It is the leader of our sense by helping us develop feelings such as fear, happiness, sorrow and all our memories. The eye, at a glance, seems so simple but there are so many fine details within the human eye. Here is a look at what is inside these magnificent windows to our world.

• The eye is approximately 1 inch (2.54 cm) wide, 1 inch deep and 0.9 inches (2.3 cm) tall.
• Pupil size can change from 2 millimetres to 8 millimetres. The eye can change the amount of light that enters it by 30 times.

Parts of the eye:

Sclera:

The tough, outermost layer of the eye. It maintains the shape of the eye. Six ocular muscles control eye movement:

• Medial rectus: moves eye towards nose
• Lateral rectus: moves eye away from nose
• Superior rectus: raises eye
• Inferior rectus: lowers eye
• Superior oblique: rotates eye
• Inferior oblique: rotates eye

Cornea:

The front sixth of this layer is clear. All light passes through this first.

Choroid (or uveal tract):

The second part of the eye. It contains the blood vessels that supply blood to the eye. The front part of this has two structures:

Ciliary Body:

A muscular area that is attached to the lens of the eye. It contracts and relaxes to control the size of the lens for focusing.

Iris:

The colored part of the eye. The color of the iris is determined by the color of the connective tissue and pigment cells. Two muscles in the iris:

• Dilator: makes the iris smaller and the pupil larger, allowing more light in
• Sphincter: makes the iris larger and the pupil smaller, allowing less light in

Retina:

The inner most layer and the light-sensing portion of the eye. Contains rod cells, which help with vision in low light, and cone cells, which are responsible for color vision and detail. 100 million rods and 7 million cones.

Macula:

Responsible for sharp, detailed vision. Contains a high concentration of cones.

Optic Disk:

Where the optic nerve and blood vessels exit the retina

Lens:

Used to fine-tune vision. Clear, bi-convex structure about 10 mm (0.4 inches) in diameter

Conjunctiva:

Covering the inside surface of the eyelids and sclera, which helps keep the eye moist. (This is where pink eye occurs) Lacrimal Glands: Produces tears

Protection:

Orbital cavity:

Bony cavity that protects the eye

Eyelids:

Protect the eye by blinking and keeping them moist

Eye lashes and eye brows:

Keep particles from entering the eye

• The eye is also covered in fat for some cushioning

Color Vision

Cone pigments:

The colour-responsive chemicals in the cones. The retinal portion of the chemical is the same, however the scotopsin is replaced with photopsins. The color-responsive pigments are made of retinal and photopsins. There are three kinds of color-sensitive pigments:

• Red-sensitive pigment
• Green-sensitive pigment
• Blue-sensitive pigment

Light

When light enters the eye, it passes through:

• Cornea
• Aqueous humor
• Lens
• Vitreous humor
• Retina

When light contacts the rods and cones in the retina, a series of complex chemical reactions occur which create electrical impulses in the optic nerve

• Rod: Segment of cell containing photosensitive chemicals
• Cone: Segment of cell containing photosensitive chemicals

The retina contains a chemical called rhodopsin - a mixture of a protein called scotopsin and 11-cis-retinal. This converts light into electrical impulses that the brain interprets as vision.

When light enters the eye, it comes in contact with the photosensitive chemical rhodopsin. Rhodopsin decomposes when it is exposed to light because light causes a physical change in the 11-cis-retinal portion of the rhodopsin, changing it to all-trans retinal. This first reaction takes only a few trillionths of a second.

Rhodopsin breaks down into several intermediate compounds, but in less than a second forms met rhodopsin II. This chemical causes electrical impulses that are transmitted to the brain and interpreted as light.

Causes of blindness

• Accident/injury
• Cataracts: The lens of your eye focuses light onto the back of your eye. This lens can become clouded.
• Diabetic Retinopathy: Can damage blood vessels in your retina. This damage can cause your blood vessels to burst causing clouding in the retina.
• Glaucoma: The ducts that drain fluid out of your eyes can become blocked, raising the pressure inside the eyeball.
• Macular Degeneration: As people begin to age, the macula can begin to decline.
• Retinitis Pigmentosa: Quick loss of the rods and cones.