Eye functioning,accommodation,binocular Flashcards
What are the three stages involved in seeing?
- Pathway of light rays and image formation
- Stimulation of photoreceptors
- Pathway and destination of nerve impulses
Explain stage one of seeing
Pathway of light rays and image formation.
As light passes from the air into the eye, it moves successively through the cornea, aqueous humour, pupil, biconvex lens, vitreous humour, and then passes through the entire thickness of the neural layer of the retina to stimulate the photoreceptors.
When light passes from one substance to another of a different density, refraction occurs. During its passage, light is refracted (bent) as it enters the cornea and on entering and leaving the lens.
The light rays converge to form a focused image on the yellow spot of the retina.
The image will be a real image, ie it is smaller than the object, upside down and reversed from left to right.
Explain stage 2 of seeing
Stimulation of the photoreceptors
In the retina the rods and cones are stimulated by light.
In both photoreceptors, photo pigments are broken down by light energy.
This breaking down generates an electrical impulse in the photoreceptors.
Explain stage 3 of seeing
Pathway of nerve impulses
The impulses from the photoreceptors travel along two layers of neurons.
The axons of the ganglia neurons form the optic nerve which leaves the eye at the blind spot and carries impulses to the cerebral cortex.
The impulses are interpreted as vision in the occipital lobe of the cerebral cortex.
What is binocular vision?
The visual fields of both eyes overlap considerably. So each eye receives a slightly different view of the object being looked at. The brain combines this information to form a single, three-dimensional image which helps to judge the distance, depth and size of the object being viewed
What is accommodation?
The process of producing a finely-focused image on the retina. Carried out by the action of the ciliary muscles which changes the shape of the lens.
The wider it is, the more the light rays are bent; the flatter the lens, the less the light rays are bent.
When does accommodation take place?
When viewing objects nearer than 6 metres.
Light from a close object diverges (spreads out) so the lens must bulge out more to bend the light rays more to focus the image on the retina.
How is accommodation brought about?
The circular, ciliary muscles contract.
This action pulls the ciliary body inwards towards the pupil.
This causes the tension on the suspensory ligament and on the lens to slacken.
Being elastic, the lens bulges outwards becoming more convex.
This causes thelight rays to bend moreand shorten the focal length.Theimageof a close object can now befocused clearly on the retina.
How do eyes adjust to view distant objects?
From distant objects the light rays travel in parallel lines and need little refraction.
Distant vision is the natural state, where the ciliary muscle is relaxed, the suspensory ligament is taut, exerting a pull on the lens so that it is thin, bringing about the little refraction that is necessary to focus light on the retina.
