These DAV Class 8 Science Notes and DAV Class 8 Science Chapter 11 Notes – The Human Eye act as excellent revision resources, particularly in preparation for board exams.
The Human Eye Class 8 DAV Notes
→ Structure of Human Eye: The human eye has nearly spherical shape. Its diameter is about 2.5 cm.
→ Cornea: The front surface of the eye is called cornea. It is whitish in colour. The space behind the cornea is filled with a liquid; called aqueous humor.
→ Iris: There is a dark coloured muscular diaphragm behind the cornea. This is called iris. The colour of iris is responsible for particular colour of the eye.
→ Pupil: This is a small circular aperture in the centre of the iris. The size of the pupil is controlled by iris. Thus, iris controls the amount of light entering the eye. Proper amount of light entering the eye makes clear image.
→ Eye lens: This is a convex lens which is made up of transparent crystalline layers. The eye lens is held in position by ciliary muscles. The ciliary muscles help in changing the curvature of the eye lens and thus the focal length of the eye lens.
→ Retina: The surface at the back of the eye is called retina. Images are formed on the retina. There are numerous light-sensitive cells in the retina. The light-sensitive cells in the retina are of two types, i.e. rod cells and cone cells. The rod cells are responsible for vision in dim light and cone cells are responsible for vision in bright light. Cone cells also give perception of colours.
→ Blind Spot: At the junction of the optic nerve and retina there are no photosensitive cells. So, no image is formed at this spot. This point is called the blind spot of the eye.
→ Working of the human eye:
The lens of the eye works like any other convex lens. The eye lens forms a real and inverted image on the retina. This activates the light-sensitive cells in the retina and these cells generate electric signals. These signals are then sent to the brain by the optic nerves. The brain interprets these signals and we are able to see an erect image which is of the same size as object.
→ Persistence of Vision: The image formed on the retina remains on the retina for about 1/16th of a second; even after the removal of the object. This phenomenon is called persistence of vision. Animations and movies take advantage of persistence of vision and we get an illusion of motion pictures.
→ Range of Vision: The human eye can clearly see a nearby object as well as a distant object. This ability of human eye is called power of accommodation. The least distance up to which we can clearly see is 25 cm for a normal individual. This is called the least distance of distinct vision or near point of the eye. The far point of the eye lies at infinity.
→ Defects of Vision:
Myopia or Near Sightedness: In this eye defect, a person is unable to see distant objects clearly. In this condition, the light rays from a distant object converge before the retina; after passing through the eye lens. Due to this, a myopic person is unable to clearly see a distant object.
→ Causes of Myopia:
(a) Excessive curvature of cornea
(b) Elongation of eyeball.
→ Correction of Myopia: Myopia can be corrected by using a concave (diverging) lens of suitable focal length.
→ Hypermetropia or Far Sightedness: In this eye defect, a person is unable to see nearby objects clearly. In this condition, the light rays from a nearby object converge behind the retina; after passing through the eye lens. Due to this, a hypermetropic person is unable to clearly see a nearby object.
→ Causes of Hypermetropia:
(a) The focal length of eye lens becomes too large
(b) Eyeball getting shortened
→ Correction of Hypermetropia: Hypermetropia can be corrected by using a convex (converging) lens of suitable focal length.
→ Cataract: In some people, the crystalline lens becomes hazy or even opaque. This happens because of the development of a membrane over the lens. This usually happens because of ageing and the condition is called cataract. Cataract can be corrected by surgery and by removing the defective lens. An artificial lens is transplanted in place of the defective eye lens.
→ Care for the Eyes:
- Have a regular check up. If advised, use the correct spectacles.
- Never look at the sun or any other powerful source of light; directly.
- Read in sufficient light.
- Do not look continuously at computer screen or TV.
- Eat a balanced diet and specially fruits and vegetables rich in vitamin A.
- Gently wash your eyes with clean water.
→ Visually Challenged Person: A person with limited vision or with complete loss of vision is called a visually challenged person. Most of the visually challenged persons develop a better sense of touch, smell or hearing than normal people.
→ Help for visually challenged persons: Many aids and resources are available to help a visually challenged person. These resources can be of two types:
→ Non-optical Aids: These include tactual aids and auditory aids.
(a) Tactual Aids: Such aids rely on the sense of touch. Braille is a script for visually challenged persons. Different letters and symbols are represented by raised dots on paper.
(b) Auditory Aids: These include audio cassettes, audio CDs, and electronic devices like computers/calculators/closed circuit television. These can convert text files into sound; with the help of a software.
→ Optical Aids: These include lenses and telescopic aids.
→ Braille System: This system was developed by Louis Braille (who himself was a visually challenged person) in 1821. The Braille system has 63 dot patterns or characters. Each pattern is composed of 9 dots; out which one or more dot can be raised in different combinations. The Braille system has made it possible for visually challenged persons to read and to write
→ Cornea : The front surface of the eyeball is called cornea.
→ Retina : The back surface of the eyeball is called retina.
→ Power of AccommodatIon : The ability of the human eye to see a distant as well as a nearby object clearly is called the power of accommodation.
→ Near Point : The minimum distance of clear vision is called the near point of eye.
→ Far point : The maximum distance of clear vision is called the tar point of eye.
→ Visually Challenged Person : A person with partial visual disability or complete visual disability is called a visually challenged person.