Eye functioning Flashcards
What 3 stages are involved in seeing
- Pathway of light rays and image formation
- stimulation of photoreceptors
- pathway and destination of nerve impulses
What happens in the first stage of “seeing”
- light passes from air to the eye
- it moves through cornea, aqueous humour, pupil,
biconvex lens, vitreous humour until reaching neutral
layer of retina - photoreceptors now get stimulated
- As the light passes through a different density its rays
get bent [refracted] on entering and leaving the lens - the light rays converge to form a focused image on
yellow spot in retina - the image will be a real image [smaller than object]
upside down and reversed from left to right
(Cornea curvature is constant =cannot change its refractive power. The Lens is highly elastic so change can occur to allow sharp focusing on image)
What happens after the pathway of light rays have formed an image
Stimulation of the photoreceptors
1. retinas rods and cones are stimulated by light
2. in both photoreceptors the photo pigments are
broken down by light energy
3. an electrical impulse is generated in the
photoreceptors
Describe the pathway of nerve impulses
- impulses from the photoreceptors travel along two
layers of neurons - Axons of the ganglia neurons form the optic nerve
- The optic nerve leaves at the blind spot and carries
impulses to the cerebral cortex
4.impulses are interpreted as vision in the Occipital
lobe
Define binocular vision
Our eyes visual fields overlap therefor the brain combines information to form a single three-dimensional image to help judge depth, distance and size of the viewed object
What is accommodation of the eye
The process by which the lens becomes more convex [fat]so that incoming diverging rays of light from objects closer than 6 metres are bent more sharply for clear focusing to take place on the retina.
more convex=more bent
less convex=less bent
When does accommodation take place
when viewing objects nearer than 6 metres
light from a close object diverges [spread] therefor a lens must bulge out more to bend the light rays more to focus the image on the retina
How is accommodation brought about
- circular ciliary muscles contract
- this pulls the ciliary body inwards towards the pupil
- tension on the suspensory ligaments and lens slacken [loosen]
- the lens bulges becoming more convex
- causing light rays to bend more and shorten the focal length
- image of the close object now focused clearly on retina
[distant vision is the natural state of our eyes]
Name the condition which causes the lens to lose its elasticity with age
presbyopia
What is the name and causes, result and remedy of
short-sightedness
MYOPIA distant objects are blurred caused by: eyeball that is too long cornea/lens too convex
results in light rays converging to form image INFRONT of retina
remedy- wear concave lenses that diverge light rays before entering the eye
[laser treatment can correct the shape of the cornea]
What is the name and causes, result and remedy of
long-sightedness
HYPERMETROPIA can see distant objects but not those close by caused by: eyeball too short curvature of lens / cornea too flat
results in light rays converging to form image BEHIND the retina
remedy- wear biconvex lenses so that light rays converge before entering the eye so that they can form an image on the retina
What is an astigmatism + how can it be treated [3]
an irregularly shaped:
cornea [corneal astigmatism]
lens[lenticular astigmatism]
treated with soft contact lenses, spectacles or refractive laser surgery
What happens if a person has lenticular astigmatism
the defect causes light rays to focus on two points = blurred image
causes of an astigmatism
hereditary
scarred from injury
eye surgery
keratoconus
