viii.1. An Introduction to Refractive Defects of vision and the Gradual Loss of Eye Accommodation

The eye may gradually lose their power of accommodation. In such conditions, the person cannot see the objects distinctly and comfortably. The vision becomes blurred due to the refractive defects of vision.

Clearing the Vision: Understanding and Correcting Common Defects of Vision with Spherical Lenses

There are mainly three common refractive defects of vision.
These are-
- (i) Myopia or near – sightedness,
- (ii)Hypermetropia or far – sightedness, and
- (iii) Presbyopia.
These defects of vision can be corrected by the use of suitable spherical lenses.
Myopia: It is also known as near-sightedness.
- A person with myopia can see nearby objects clearly but cannot see distant objects distinctly.
- A person with these defects of vision has a far point nearer than infinity.
- In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself.
These Defects of vision may arise due to –
- (i) excessive curvature of the eye lens, or
- (ii) elongation of the eyeball.
These defects of vision can be corrected by using a concave lens of suitable power.
A concave lens of suitable power will bring the image back onto the retina and thus the defects of vision are corrected.

Hypermetropia: It is also known as far-sightedness.
- A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly.
- The near point, for the person, is farther away from the normal near point (25 cm).
- This is because the light rays from a closeby object are focussed at a point behind the retina as.
These defects of vision arises either because-

(i) the focal length of the eye lens is too long, or

(ii) the eyeball has become too small.

These defects of vision can be corrected by using a convex lens of appropriate power.
Eye-glasses with converging lenses provide the additional focussing power required for forming the image on the retina.

Presbyopia: The power of accommodation of the eye usually decreases with aging.
- For most people, the near point gradually recedes away.
- They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. These defects of vision are called Presbyopia.
- It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.
- Sometimes, a person may suffer from both myopia and hypermetropia.
- Such people often require bi-focal lenses.
- A common type of bi-focal lens consists of both concave and convex lenses.
- The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision.

Overcoming Defects of Vision Challenges: The Remarkable Ability of Visually Impaired Individuals to Read and Write

Some persons, including children, can be visually handicapped.
They have very limited vision to see things. Some persons cannot see at all since birth.
Some persons may lose their eyesight because of a disease.
Such persons try to identify things by touching and listening to voices more carefully. They develop their other senses more sharply, showcasing remarkable adaptability in the face of defects of vision.
However, additional resources can enable them to develop their capabilities further.

Louis Braille, himself a visually challenged person, developed a system for visually challenged persons and published it in 1821.
The Braille system has 63 dot patterns or characters. The present system was adopted in 1932.
There is Braille code for common languages, mathematics and scientific notation.
Many Indian languages can be read using the Braille system.

EYES DONATION

Our eyes can live even after our death. About 35 million people in the developing world are blind and most of them can be cured.
About 4.5 million people with corneal blindness can be cured through corneal transplantation of donated eyes. Out of these 4.5 million, 60% are children below the age of 12.
Eye donors can belong to any age group or sex.
People who use spectacles, or those operated for cataract, can still donate the eyes.
People who are diabetic, have hypertension, asthma patients and those without communicable diseases can also donate eyes.
Eyes must be removed within 4-6 hours after death. Inform the nearest eye bank immediately.
The eye bank team will remove the eyes at the home of the deceased or at a hospital.
Eye removal takes only 10-15 minutes. It is a simple process and does not lead to any disfigurement.
Persons who were infected with or died because of AIDS, Hepatitis B or C, rabies, acute leukaemia, tetanus, cholera, meningitis or encephalitis cannot donate eyes.
An eye bank collects, evaluates and distributes the donated eyes. All eyes donated are evaluated using strict medical standards.
Those donated eyes found unsuitable for transplantation are used for valuable research and medical education.
The identities of both the donor and the recipient remain confidential. One pair of eyes gives vision to up to four corneal blind people.

Did you know?

Animals have eyes shaped in different ways.
Eyes of a crab are quite small but they enable the crab to look all around.
So, the crab can sense even if the enemy approaches from behind. Butterflies have large eyes that seem to be made up of thousands of little eyes, highlighting their unique visual structure that aids them in navigating their surroundings, free from common defects of vision.
They can see not only in the front and the sides but the back as well.
A night bird (owl) can see very well in the night but not during the day.
On the other hand, daylight birds (kite, eagle) can see well during the day but not at night.
The owl has a large cornea and a large pupil to allow more light in its eye.
Also, it has on its retina a large number of rods and only a few cones.
The day birds on the other hand, have more cones and fewer rods.

Empowering Vision: Navigating the World of Non-Optical and Optical Aids for Overcoming Defects of Vision

Non-optical aids include visual aids, tactual aids (using the sense of touch), auditory aids (using the sense of hearing) and electronic aids.
Visual aids, can magnify words, can provide suitable intensity of light and material at proper distances.
Tactual aids, including Braille writer slate and stylus, help the visually challenged persons in taking notes, reading and writing.
Auditory aids include cassettes, tape recorders, talking books and other such devices.
Electronic aids, such as talking calculators and computers, are also available for performing many computational tasks.
Closed circuit television, also an electronic aid, enlarges printed material with suitable contrast and illumination.
Nowadays, use of audio CDs and voice boxes with computers are also very helpful for listening to and writing the desired text.
Optical aids include bifocal lenses, contact lenses, tinted lenses, magnifiers and telescopic aids.
While the lens combinations are used to rectify visual limitations , telescopic aids become crucial for individuals grappling with defects of vision, telescopic aids are available to view chalkboard and class demonstrations.

Some visually impaired Indians have great achievements to their credit.
Diwakar, a child prodigy has given amazing performances as a singer.
Ravindra Jain, born completely visually impaired, obtained his Sangeet Prabhakar degree from Allahabad.
He had shown his excellence as a lyricist, singer and music composer.
Lal Advani, himself visually impaired, established an Association for special education and rehabilitation of disabled in India.
Besides this, he represented India on Braille problems in UNESCO.
Helen A. Keller, an American author and lecturer, is perhaps the most well known and inspiring visually challenged person.
She lost her sight when she was only 18 months old. But because of her resolve and courage she could complete her graduation from a university.
She wrote a number of books including The Story of my Life (1903).