Class 8 Science

Human Eye

Eyes are the sense organs which give us the sense of vision. The human eye is almost spherical in shape. Following are the main structures in the human eye.

structure of human eye

Cornea: The eyeball is covered with a tough layer. This layer is transparent on the front. This transparent portion is called cornea.

Iris: Iris is a thin circular structure. It works like the shutter of a camera. It controls the amount of light entering the eye. The colour of iris imparts distinct colour to the eyes of an individual. A person with blue iris has blue eyes.


Pupil: There is a hole in the centre of iris. This is called pupil. Light enters the eye through the pupil. When the light is bright, iris contracts and thus allows less light into the eye. When the light is dim, iris dilates and thus allows more light into the eye.

Lens: Lens is present behind the pupil. The lens in the human eye is a convex lens. The size and thickness of the lens change as per the distance of an object.

Retina: Retina is at the back of the eye and marks the inner layer of the eyeball. Images are formed on the retina and thus it works like a screen. There are photosensitive cells in the retina. These cells are of two types, which are as follows:

  • Cone Cells: The cone cells are sensitive to bright light. They also give the sense of colour.
  • Rod Cells: The rod cells are sensitive to dim light.

Optic Nerve: The optic nerve emerges from the back of the eye ball. This nerve goes to the brain.

Blind Spot: The junction of retina and the optic nerve is called the blind spot. There is no photosensitive cell at this spot, and hence no image is formed at this spot on the retina.


Persistence of Vision

An image stays on the retina for about 1/16 of a second. This feature is called persistence of vision. Due to this, when many still images are shown in a sequence; they give the illusion of moving images. Movies and animation are made by exploiting this property of the human eye.

Care of Eyes

Eyes are very important organ and they are sensitive too. So, proper care of eyes is very important. Some tips for care of eyes are as follows:

  • Do not read in too bright or too dim light.
  • Do not look directly at a bright object or at the sun.
  • Do not keep the book too close to your eyes; while reading. Don’t keep the book too far either.
  • If something gets into the eye, do not rub the eye. Wash it with cold water.
  • In case of any problem; like itching or burning sensation; consult an ophthalmologist. A doctor who specializes in the disease of eyes is called an ophthalmologist.

Vision Defects:

Some of the vision defects are as follows:

Myopia: A person suffering from myopia finds it difficult to see a distant object. This disease is also called short sightedness or near sightedness. Spectacles with suitable lenses can improve the vision in a myopic person.

Hypermetropia: A person suffering from hypermetropia finds it difficult to see a nearby object. This disease is also called long sightedness. Spectacles with suitable lenses can improve the vision in a hypermetropic person.

Cataract: The cornea becomes cloudy in a cataract patient and thus the person cannot see clearly. Cataract is treated by surgery. In cataract surgery, the cornea is cleaned and an artificial lens is transplanted in the eye.


Visually Challenged Person:

Some people face with disability of vision. This disability can be partial or complete. Such persons are called visually challenged persons. For a visually challenged person; life can be very difficult. These people usually show a marked development of other senses; like the sense of hearing and sense of touch. Many aids have been devised to make their life easy. They can be divided into two categories, viz. optical and non-optical aids.

Optical Aids: Optical aids can help a person who is partially visually challenged. These aids enlarge an image or a text so that they could be visible. TV monitors, magnifying devices and telescopic devices come under this category.

Non-optical Aids: Non-optical aids are helpful for a person who is completely visually challenged. These aids rely on the senses of hearing and touch. Aids which rely on the sense of touch are called tactual aids. Tactile buttons on the pedestrian light and in public transport are examples of tactual aids. Tactile strips at the edge of the platforms are also meant for visually challenged persons. Even the currency notes have tactile markings so that a visually challenged person can recognize notes of different denominations.

In some countries; specially trained guide dogs are pressed into the service of a visually challenged person.

Louis Braille Louis Braille was born in 1809 in France. He became blind during early childhood because of an accident. He developed the Braille script in 1824.

Braille Script: The Braille scripts are written in the form of raised dots. This system is composed of 63 characters. Each character is written on a grid of 6 cells. This can be written with the help of Braille slate and stylus. A reader needs to touch the Braille script to read it.