IGCSE Biology CH 14B The Eye Extended PDF

BY CELL :00201110983031 00201110983031 [email protected]  1 Describe sense organs as groups of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals 2 Identify in diagrams and images the structures of the eye, l...

BY CELL :00201110983031 00201110983031 [email protected]  1 Describe sense organs as groups of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals 2 Identify in diagrams and images the structures of the eye, limited to: cornea, iris, pupil, lens, retina, optic nerve and blind spot 3 Describe the function of each part of the eye, limited to: (a) cornea – refracts light (b) iris – controls how much light enters the pupil (c) lens – focuses light on to the retina (d) retina – contains light receptors, some sensitive to light of different colours (e) optic nerve – carries impulses to the brain 4 Explain the pupil reflex, limited to changes in light intensity and pupil diameter CELL :00201110983031 00201110983031 [email protected]  THE EYE The human eye is a sensory organ. This means it is an organ of tissues working together to detect and respond to a specific stimulus, which is light. Found in a groove in the skull called orbit or socket. It moves in its orbit by three pairs of muscles called eye muscles. It is protected by: Skull, eye lids, lashes and tears. Components of tears Water, sodium bicarbonate, sodium chloride and enzyme called lysozyme. Functions of tears 1. Reduces friction between lids and eye 2. Lysozyme kills microbes. 3. Wash the eye Functions of the different structures of the eye Sclera (sclerotic coat) 1. muscle fiber Protects the eye. 2. Attached to the eye muscles which move the eye in its orbit. 3. does not envelop all the eye because it is opaque, therefore its front part is transparent and is called cornea. Cornea Has a curved surface where most refraction of light happens, so that light rays are collected to enter the eye. CELL :00201110983031 00201110983031 [email protected]  Two important regions found in the retina, fovea or yellow spot and blind 1. Fovea Or Yellow spot the point of accurate vision because it contains most cones 2. Blind spot: 1. The point where optic nerve and blood vessels leave the eye. 2. it has such name because it contains no light receptors therefore it cannot detect images CELL :00201110983031 00201110983031 [email protected]  How to determine blind spot? 1. put the book about 50 cm away 2. close the left eye and concentrate on the cross with the right Eye. 3. Slowly bring the book closer to the face. 4. When the image dot falls on the blind spot it will seem to disappear. Notice If you look straight at an object in dim light you cannot see it clear as it falls on fovea that contain cones which cannot detect dim light but you can see it clear if you look at one side asthe image falls on rods. Optic nerve: Carries impulses from the eye to the centre of vision in the brain. Vitreous humour:  Jelly-like fluid Keeps shape of eye.  Supports retina to be attached to the choroid-Helps refraction (bending) of light to be focused on the retina. Aqueous humor Saline solution contains salt Supply food to the lens and cornea Helps refraction bending of light. Iris Controls the amount of light that enters the eye Pupil An opening in the iris ,It is the pathway of light into the eye. Eye Lens Adjust (focus) the light rays on the retina. CELL :00201110983031 00201110983031 [email protected]  Important comparison How iris controls the amount of light that enters the eye In dim light In bright light 1. Radial muscles of the iris contract 4. -Radial muscles of the iris relax 2. Circular muscles relax 5. Circular muscles contract 3. Pupil becomes larger so that enough - Pupil becomes smaller to light can enter the eye. reduce the amount of light that enters the eye to protect the light receptors CELL :00201110983031 00201110983031 [email protected]  RODS CONES 1. Larg in number 1. Smaller in number. 2. Spread in all retina but much less in 2. present in fovea mostly. fovea and not found in blind spot. 3. Sensitive to bright light. 3. Sensitive to low light intensity 4. Detect colors. therefore can. 5. Each has its own nerve connection 4. detect dim light. therefore it is more accurate. 5. Cannot detect colours. 6. There are three types of cones each 6. Each group share one nerve type is specific in certain wave length. connection therefore it is less accurate in vision. CELL :00201110983031 00201110983031 [email protected]  NEAR OBJECT FAR OBJECT 1. Ciliary muscles contract 2. A Suspensory ligaments slacken. 1. Ciliary muscles relax 3. The eye lens becomes thicker. 2. Suspensory ligaments become 4. Pupil becomes small stretched 3. The eye lens becomes thinner 4. Pupil becomes larger. CELL :00201110983031 00201110983031 [email protected]