1. The eyes are the sensory organs of sight. They are sensitive to light.
2. The eyelids and eyelashes keep dust and dirt out of the eyes.
3. The eye brows help to prevent sweat from running into the eyes.
4. The wall of eyeball is made up of three layers.
(a) Sclere – tough, outer layer
(b) Choroid – black, middle layer
(c) Retina – sensitive to light , innermost layer
| The structure of the human eye and the function of the different parts Part | Stucture | Function |
| Sclera | White, fibrous, outer layer that is opaque and tough | Protects and maintains the shape of the eyeball |
| Choroid | Middle layer that contains black pigment and many blood vessels | - The blood vessels supply nutrients and oxygen to the eye. - The black pigment absorbs light and therefore prevents reflection of light inside the eye which would make an image less sharp. |
| Retina | The innermost layer of the eyeball. It contains many photoreceptors known as cones and rods | - Detect light and produces nerve impulses - The cones detect colours in bright light, enabling us to have colour vision -The rods enables us to see in dim light . They detect shades of grey in dim light. |
| Cornea | The curved and transparent layer of the sclera at the front of the eye. | Its curved surface helps to refract (bend) light onto the retina. |
| Conjunctiva | The thin , transparent layer of membrane in front of the cornea | Protects the cornea |
| Iris | The coloured part of the eye that is continous with the choroids. It is made up of muscles | Controls the size of the pupil and thus the amount of light entering the eye. |
| Pupil | The hole in the centre of the iris | Controls the amount of light entering the eye. |
| Lens | A transparent bioconvex and leastic disc. | -It refract and focuses an image onto the retina. -The thickness of the lens is changed to focus near and distant objects. |
| Ciliary body | Make up of strong muscle | Contract and relaxes to change the thickness of the lens |
| Suspensory ligaments | Strong fibres that connect the lens to the ciliary body | Hold the lens in its position |
| Aqueous humour | A watery transparent liquid that fills the space between the cornea and the lens | - Helps to focus the image onto the retina -Helps to maintain the shape of the eyeball -The cornea , the conjunctiva and the lens obtain food and oxygen from the blood vessels in the choroids layer by diffusion through this liquid. |
5. How wee see
i) When we look at an object, light rays from the object enter each eye through the pupil. The intensity
of the light influences the size of the pupil.
a) When it is very bright , the size of the pupil decreases to let in less light.
b) When it is dim, the size of the pupil increases to let in more light.
ii. As the light rays pass through the cornea, the aqueous humour , the lens and vitreous humour, the
rays are refracted to form an image on the retina.
iii. The image is upside down and smaller than the actual object.
iv. The brain interprets the impulses and we can see the object the right way up. The
interpretation also allows us to know the shape, the colour , the size and the distance of the object.
v. Eyes have the ability to focus on objects a different distance.
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