1. How is the human eye adapted to its function?

• conjunctiva is thin/transparent/tough to allow light to pass through/to protect the eye
• Sclerotic layer is made up of (collagen) fibers/fibrous. It maintains shape of the eyeball/protects the eye
• cornea is transparent/curved thus refracts light rays/allows light to pass through
• Choroid is a layer of tissue with black pigment/dark pigment. Prevents internal reflection of light in the eye/contains blood vessels that supply oxygen/nutrients/remove (metabolic) wastes from the eye
• retina has cones/rods for bright colour vision/low light vision
• yellow spot has a high concentration of cones for accurate vision/visual acuity
• Blind spot has no cones and rods. Place where optic nerve leaves/enters the eye
• optic nerve has (sensory) nerve fibers for transmission of impulses to the brain (for interpretation)
• Lens is biconvex/made up of elastic material/transparent. Adjust focus on far or near objects allow light to pass through/for refraction of light rays
• ciliary body is made up of muscle fibers/glandular which contract/relax to change shape
• suspensory ligaments are inelastic to hold lens in position/attach it to ciliary body
• iris(is the coloured part of the eye it) has radial and circular muscles which control size of pupil
• pupil is the small hole at the centre of iris through which light passes into the eye
• aqueous humor is a fluid through which oxygen/nutrients pass to the cornea/lens/maintains shape of the eyeball/refracts light rays
• vitreous humor is a fluid which maintains shape of eye/refracts light rays

iii) What is accommodation of the eye?

• ability of the eye to adjust to bring an image from a near or far object into sharp focus on the retina

iv) Explain how an eye viewing a near object adjusts to viewing a far object

• ciliary muscles relax
• suspensory ligaments become taut/tight
• lens decreases curvature/becomes thinner
• radial muscles relax
• circular muscles contract
• size of pupil decreases to reduce amount of light

v) What changes occur in the eye if it changes from observing an object at a distance to one at a closer range?

– ciliary muscles contract

– Tension in suspensory ligaments reduces/relax/ slackens

– Lens bulges/thickens/increases curvature

– Radial muscles contract

– Circular muscles relax

– Size of pupil becomes large to allow in more light.

1. State the changes which would take place in the eye if a person in a dark room had lights switched on
2. circular muscles contract and radial muscles relax
3. pupil becomes small to allow less light into the eye
4. Explain how the eye forms an image
5. the mammalian eye works like a camera
6. light rays enter the cornea pass through the pupil, aqueous humor, lens and vireous humor
7. light rays are refracted by the aqueous and humor and lenses
8. finally light falls on the retina to form an image
9. the image is real and inverted and smaller than object, back to front/reversed
10. Retina forms a fine image when light rays reach it.
11. Name the defects of the eye and state how they can be corrected

Short sight (Myopia)

• eye cannot focus on far objects
• image is formed in front of the retina because light rays converge in front of retina
• the lens is too thick, curve and eyeball too long
• corrected by wearing concave/biconcave/lenses

these lenses diverge light rays onto retina

Long sight (Hypermetropia)

• eye lenses are unable to focus because they are flat, thin and weak hence unable to focus image on the retina
• they are unable to accommodate/change the focal length
• near image is formed behind the retina but a distant one is correctly focused on the retina
• corrected by wearing convex/biconvex/converging lenses

Presbyopia

• occurs in old age hence called old sight
• caused due to loss of elasticity of lenses, weakness of ciliary muscles hence lack of focus of light rays
• this causes long sight
• corrected by wearing biconvex/convex/converging lenses

Squinting

• eyeballs are uncoordinated/do not turn at the same time
• eye muscles move in different directions
• this makes accommodation and focusing difficult
• corrected through surgery

Astigmatism

• surface of cornea is uneven
• leads to weak focus of light raise on retina
• corrected by using cylindrical lenses/lenses with combined curvature
• State the advantages of having two eyes in human beings
• stereoscopic vision
• gives a wider angle of binocular vision
• if one is damaged human is not blinded