The Eye

Anatomy of the Eye

• The eye is an organ that enables vision. It captures light and sends signals to the brain that interprets them as images.
• Main parts of the eye include the cornea, pupil, lens, retina, optic nerve, and sclera.
• The cornea is the transparent layer covering the front of the eye that helps focus light entering the eye.
• The pupil is the hole in the middle of the iris through which light enters the inner eye.
• The lens focuses light onto the retina.
• The retina is the light-sensitive lining at the back of the eye containing rod and cone cells.
• The optic nerve transports visual information from the eye to the brain.
• The sclera is the tough outer layer (the ‘white’ of the eye) protecting the sensitive inner parts.

The Process of Vision

• When light enters the eye, it passes through the cornea, pupil, and lens before reaching the retina.
• The lens adjusts its shape to focus light onto the retina, a process called accommodation.
• On the retina, light stimulates rods and cones, the photoreceptor cells.
• Rods are more sensitive in dim light and provide black-and-white vision.
• Cones require bright light and are responsible for colour vision and sharpness (acuity).
• The stimulated photoreceptor cells produce electrical signals which are then transmitted to the brain via the optic nerve.
• The brain (via the visual cortex in the cerebrum) interprets these signals as images.

Adjustments for Light Levels

• The iris controls the amount of light entering the eye by changing the size of the pupil.
• In bright conditions, the pupil constricts (gets smaller) to reduce the amount of light entering.
• In dim conditions, the pupil dilates (gets larger) to allow more light in.

Focusing on Near and Distant Objects

• To focus on distant objects, the ciliary muscles relax which causes the lens to thin.
• To focus on close objects, the ciliary muscles contract, making the lens thicker and increasing its refractive power.

Eye Problems and Correction

• Short-sightedness (myopia) occur when the eye over-focuses and the image is formed before the retina; a concave lens is used to correct this.
• Long-sightedness (hypermetropia) is when the eye under-focuses and the image is formed behind the retina; a convex lens is used to correct this.
• Colour blindness, usually inherited, is the inability to distinguish certain colours and is due to deficiencies in certain types of cones.