Working of the Eye
In working of eye when light enter into the eye it passes through pupil. Iris controls the amountof the light. The cilliary muscle tissues help the lens to focus the object on the retina. When lightstrikes on either on the rods or the cones of the retina, it is converted into an electric signal. The photoreceptors in the retina convert the light into the electrical signals. These signals carried to the brain by optic nerves and nerve fibers. The brain then translates the electrical signal into photographs or image we see.
Working of human eye has two functional parts : dioptric or focussing part and receptor part.
(a) Focussing Part. It consists of conjunctiva, cornea, aqueous humour, lens and vitreous humour. These parts are transparent and act as lenses. They refract the light rays passing through the eye to bring them to a focus on the retina . Maximum refraction is caused by the cornea, which places the image approximately on the retina. The lens effects fine adjustment and brings the image into a sharp focus.
(b) Receptor Part. It comprises the retina. The image formed on the retina is inverted and smaller. It converts the energy of specific wavelengths of light into receptor (action) potentials (sensory impulses) of nerve fibres. The nerve impulses are carried by the optic nerve to the visual areas of cerebral hemispheres where the real perception of sight arises and one sees the object upright.
Accommodation is the reflex mechanism by which the focus of the eye changes to make the images of distant and near objects sharp on the retina. Human eye has a good power of accommodation. A normal eye can accommodate light from objects from about 25 cm to infinity. Accommodation requires refraction (bending) of light rays to focus them on the retina. Refraction occurs when light passes from one medium to another having a different refractive index. In the eye, refraction takes place at the air-corneal surface and at the lens. The degree of refraction at the corneal surface cannot be varied as it depends on the angle at which the light strikes the cornea and this, in turn, depends upon the distance of the object from the cornea. Therefore, the degree of refraction is changed by changing the convexity of the lens. This is done with the help of ciliary muscles and suspensory ligament, which are said to form the accommodation apparatus .
(i) Distant Objects. When focussed for seeing distant objects (those beyond 6 metres), the said to be at rest. Light rays from distant objects are parallel when they strike the eye. At this time, the ciliary muscles are fully relaxed, the suspensory ligament is under maximum tension (peripheral pull), and the lens is flattened.
(ii) Near Objects. Light rays from near objects (those within 6 metres) are diverging when they strike the eye. Therefore, greater refraction of light is needed for focusing the near objects. To increase refraction, the convexity of the lens is increased by reducing the tension in the suspensory ligament. The tension in the ligament is reduced by contraction of the circular and meridional ciliary muscles. Contraction of circular muscles shortens the radius of suspensory ligament, thus making it loose. Contraction of meridional muscles pulls the choroid forward. This also makes the ligament loose. Loosening of suspensory ligament allows the lens to shorten its diameter by its own elasticity, thus becoming thicker and more convex. Increase in the thickness of the lens shortens its focal length and adjusts it to focus the near objects . Reverse occurs to see the distant objects.
Man has binocular vision in which both the eyes are focussed on the same object but from slightly different angles. The visual fields of both eyes overlap and the foveae of both are focused on the same object. This provides depth to the images, i.e., gives stereoscopic or 3D effect and enables man to judge distances correctly.
Vision in Other Animals.
Primates and predatory animals, such as owl and cat, have binocular vision. In some animals, such as rabbit, birds, each eye is focussed on a separate object. This is termed monocular vision.
Colour Vision. It is the ability of some animals to detect colours in an object. Humans, apes, monkeys, and most fishes, amphibians, reptiles and birds have strong colour vision. The insects and crayfish also have colour vision. Cats have limited colour vision. In vertebrates, colour vision results from the activity of cone cells. Most domestic and nocturnal mammals and sharks lack colour vision. They probably see objects in shades of grey (monochrome vision).
Nocturnal and Diurnal Vision. Man has both day vision and night vision as he has both rods and cones in considerable numbers in the retina. Most birds have only day vision as their retina contains mainly cones. Owls have much better night vision than day vision for they possess a large number of rods and few cones in their retina.
Range of Vision. The visible range of spectrum varies in animals. Bees, ants, spiders and goldfish can see ultraviolet light, which is invisible to man.
Vitamin A and Eyesight. Vitamin A is essential for good eye sight as it forms retinal,a component of visual pigments in the rods and cones. Deficiency of vitamin A causes night blindness (nyctolopia).
Care of Eyes
The eyes are very delicate organs and need special care. The following precautions are suggested for their safety-
1. Eyes, particularly of children, should be got periodically examined for eye defects. If any defect is found, it should be immediately corrected.
2. While reading, the page should be held about 36 cms. from the eyes and preferably at an angle of 45-70° from the horizontal.
3. One should read in the proper light that does not strain the eyes. Light should preferably come from the left side, not from the front.
4. The small print that strains the eyes should be avoided.
5. Eyes should be protected from dust, smoke, poisonous gases and very strong light.
6. Foreign objects, such as dust, coal or sand particles, hay pieces, etc., may fall into the eyes. Eyes should not be rubbed to dislodge them as this may injure the cornea if the objects are angular. Washing the eyes with clean water, may remove them, else the help of a physician should be taken.
7. Eye infections, such as trachoma, eye flu, should be taken seriously and promptly treated. They may cause blindness and also spread to others in the family.
8. Any accidental injury to the eye must be quickly attended to by an eye specialist.
9. Nothing should be put into the physicians advice. Quack’s eye lotions can be harmful.
Prevention of Eye Infection.
Eye infection can be checked by the following steps-
- A child with sore eyes should not be sent to the school as infection can spread to other children.
- Flies should not be allowed to sit on the sore eyes as they can carry infection to healthy eyes of others.
- Persons with eye infections should have separate handkerchief, towel, pillow, etc. Use of these articles by others can give infection.
- Eye discharge of the child should not be wiped off by the mother with her clothes. A separate neat napkin should be used for this and it should be frequently changed to avoid reinfection.
- Eye should not be touched with unclean hands or clothes.
Disorder or defect of eye and their correction
Disorder of eye :
Defects of vision is the loss of power of accommodation of the human eye. They are mainly following types are given below
(i) Near-sightedness (Myopia). The eyeball is longer than normal. Light rays converge in front of the retina, causing a blurred image.
Correction : This condition can be corrected by using appropriate concave lenses.
(ii) Far-sightedness (Hypermetropia). The eyeball is shorter than normal. Light rays strike the retina before converging, causing a blurred image.
Correction: This condition can be corrected by using appropriate convex lenses.
(iii) Astigmatism. This condition is due to irregular curvature of cornea or lens.
Correction: It can be corrected by using cylindrical glasses.
(iv) Presbiopia. This condition is due to loss of flexibility of the lens. It creates difficulty in focussing on near objects.
Correction: It can be corrected by using convex lenses.
(v) Cataract. Lens becomes opaque due to disease or ageing. It leads to blindness.
Correction : It can be corrected by removing the lens and wearing suitable glasses, or by replacing the defective lens with a normal lens from a donor.
Cornea may also becomes opaque. This is a serious condition, It is corrected by grafting a new cornea.
People should “will” their corneas and lenses to others at the time of death. Because the lens and the cornea have little blood supply, tissue rejection is not a serious problem with their transplant.
(vi) Glaucoma. Over-production of vitreous humour increases pressure in the eye and this crushes the delicate cells of the retina, causing blindness. This condition is called glaucoma.