Sight - The Eye

Part	Function
Conjunctiva	Thin membrane protecting the sclera. Inflammation leads to conjunctivitis.
Sclera	Tough, white, outer coat that holds the eye in shape.
Cornea	Transparent section of the eye that allows light to enter the eye.
Choroid	Dark-brown or black layer containing blood vessels that nourish the eye. Absorbs light to prevent reflection.
Retina	Light-sensitive layer containing rods and cones. Rods detect black and white, while cones detect colour.
Fovea	Central part of the retina with only cones, providing the sharpest vision and focus.
Blind spot	Area with no rods or cones, so no images are detected.
Optic nerve	Carries impulses from the eye to the brain.
Lens	Elastic, transparent structure that changes shape to focus light on the retina.
Ciliary muscle	Surrounds the lens and adjusts its shape to focus on objects at varying distances.
Iris	Coloured part of the eye. Controls the amount of light entering the eye.
Pupil	Opening in the iris that appears black because no light emerges from the inside of the eye.
Aqueous humour	Liquid that maintains the shape of the front part of the eye.
Vitreous humour	More viscous fluid that maintains the shape of the eyeball by exerting outward pressure.

Pupil Reflex

• Bright light: the pupil contracts to reduce the amount of light entering the eye.
• Dim light: Pupil dilates (enlarges) to allow more light to enter the eye.
• When we observe an object that we admire, our pupils also enlarge.

Vision Problems

Long Sight (Hyperopia)

• A person can see distant objects clearly but cannot see near objects clearly.
• Caused by the eyeball being too short or the ciliary body being too weak.
• Corrected by placing a convex lens in front of the eyes.

Short Sight (Myopia)

• A person can see near objects clearly but cannot see distant objects clearly.
• Caused by the eyeball being too long or the ciliary body being too strong, resulting in the image being focused in front of the retina.
• Corrected by placing a concave lens in front of the eyes.

The Eye Focuses Light

• Light waves travel in straight lines. These waves must be bent or refracted so that they focus exactly onto the retina.
• Light waves are bent by the cornea and lens to focus the light onto the retina.
• The shape of the lens can be changed to allow nearby or distant objects to be viewed. The process of changing the focus of the eye is called accommodation.
• The light receptor cells (cones and rods) in the retina use light to generate impulses in sensory neurons.
• These impulses are transmitted to the brain via neurons in the optic nerve.
• The image on the retina is upside down. The brain interprets this so that we see the right way up.