Parts of the eye
Iris |
Controls the amount of light entering the eye |
Pupil |
A hole in the centre of the iris where light enters |
Cornea |
Transparent layer, refracts light rays into pupil |
Humour |
Refracts light rays (aqueous/vitreous) |
Lens |
Transparent, circular, biconvex structure, elastic, change shape to refract and focus light onto the retina |
Ciliary body |
Contains ciliary muscles that contract/relax to control the curvature of the lens |
Choroid |
Pigmented black to prevent internal reflection of light, contains blood vessels to bring nutrients/oxygen and remove metabolic wastes |
Retina |
Cones; colours in bright light. Rods; black and white in the dark |
Fovea |
Largest concentration of cones, where images are usually focused |
Blind spot |
Devoid of photo-receptors and is insensitive to light |
Eyelids |
Protects eye from physical damage, prevents excessive light from damaging tissues inside the eye |
Sclera |
Tough, white outer covering, protects the eyeball from physical damage, covered by conjuctiva |
Tear glands |
Secretes tears to wash away dust particles, lubricate the eye, keep conjuctiva and cornea moist |
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Pupil reflex
Reflex action is an involuntary response to a specific stimulus, without conscious control. It involves the brain as the reflex centre.
Pathway: Stimulus -> photo-receptors in retina -> sensory neurone in optic nerve -> brain -> motor neurone -> effector |
Distance
1 Focusing on a distant/near object
2 Ciliary muscles relax/contract
3 Suspensory ligaments become taut/slacken
4 Pulling/relaxing on the edge of the lens
5 Lens become less/more convex, increasing/decreasing focal length
6 Light rays from the object are sharply focused on the retina
7 Photo-receptors are stimulated
8 Nerve impulses produced are transmitted by the optic nerve to the brain, which then interprets the impulses and the person sees the object.
Distant; ciliary relax, lens becomes thinner
Near; ciliary contract, lens becomes thicker |
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Brightness
1 Change in light intensity
2 Stimulus is detected by photo-receptors located in the retina
3 An electrical impulse is generated, transmitted by sensory neurones in the optic nerve to the brain
4 At the brain, the impulse is transmitted across a synapse to the relay neurone, then across another synapse to the motor neurone
5 Motor neurone transmits the impulse to the circular and radial muscles of the iris
6 Circular muscles contract/relax, while radial muscles relax/contract. This causes the pupil to constrict/dilate, thereby reducing/increasing the amount of light entering the eye
Bright: Circular contract, radial relax, pupil constrict
Dim: Circular relax, radial contract, pupil dilate |
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