What Are The Defects Of Vision
The major defects of vision are:
- Short sightedness or myopia.
- Long sightedness or hypermetropia.
- Presbyopic
- Astigmatism
1. Short sightedness or myopia
Symptoms: Eye cannot see clearly beyond a certain distance. It means that the far point of the defective eye has shifted from infinity to a finite distance ahead.
Reasons: It is so because the image of distant objects is formed in front of the retina. It is shown in fig.
Causes:
(i) The lens may be thicker (more converging) that the normal eye lens.
(ii) The eye ball may be elongated,
Fig. Elongated eye.
Correction: The extra converging power of eye lens is compensated by using a concave (diverging) lens of proper power (focal length) as shown in fig.
Calculation: Let distance of far point F’ from eye = x. Then for lens to be used,
u = ∞, v = –x, f = ?
From lens formula,
\( \frac{1}{v}-\frac{1}{u}=\frac{1}{f} \)
\( \frac{1}{-x}=\frac{1}{f} \)
or f = –x
The lens used must have focal length equal to the distance of the far point from the eye (–ve sign means concave lens).
2. Long sightedness or hypermetropia
Symptoms: With this defect eye cannot see clearly within a certain distance. It means that the near point of the defective eye has shifted from 25 cm to some more distance behind (away).
Reason: It is so because the image of near objects is formed behind the retina. It is shown in fig.
Hypermetropic eye vision.
Causes:
(i) The eye lens may be thinner (less converging) than the normal eye lens.
(ii) The eye ball may be oval distance between lens and retina becomes less than that for normal eye.
Fig. Oval eye.
Correction: The deficiency in converging power of eye lens is compensated by using a convex (Converging) lens of proper power (focal length) as shown in fig.
Fig. Hypermetropia corrected by a convex lens.
Calculation: Let distance of near point N’ from eye = y. Then, for lens to be used
u = –D, v = –y, f = ?
From lens formula,
\( \frac{1}{v}-\frac{1}{u}=\frac{1}{f} \)
\( \frac{1}{-y}-\frac{1}{-D}=\frac{1}{f} \)
\( f=\frac{yD}{y-D}\text{ }\left( \text{Which is positive }\because \text{ }y\text{ }>\text{ }D \right) \)
f = (Which is positive ∵ y > D)
This is required expression for the focal length of the convex lens to be used.
3. Presbyopic
The power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. They find it difficult to see nearby objects comfortably and distinctly without corrective eye-glasses. This defect is 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 lenses 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. These days, it is possible to correct the refractive defects with contact lenses or through surgical interventions.
4. Astigmation:
A person suffering from this defect cannot simultaneously focus on both horizontal and vertical lines of a wire gauze.
Normal Wire Gauge
Wire gauge with distorted vertical lines
Wire gauge with distorted horizontal lines
This defect arises due to the fact that the cornea is not perfectly spherical
This defect can be corrected by using cylindrical lens
Cylindrical lens
Alternate Technology Available for the Blind
Till now, very limited resources were available for blind people, the most popular one being the traditional Braille. The Braille system, which enables the blind to read and write, uses patterns of raised dots to represent letters and numerals. With advancement of technology, there is a marked improvement in the kind of material available for the blind people.
- Devices like portable Braille typewriter, talking calculator, talking clock, audio dictionary, dictaphone, drawings that are raised, and audio measuring devices are very useful to the blind.
- Personalized blind-friendly computers equipped with software specially suited for the blind are available today. These make accessing information available on the Internet much easier for the blind. Reading software like Jaws and Kurzweil 3000 are also very useful for the blind.
- Special keyboards based on the Braille system, with characters embossed on them, are available. Special scanners and printers are also available today, which have made accessing information very easy for the blind people.
Nutrition and Eye Health
Eye health has a very strong relationship with the food we eat. Eating a diet deficient in Vitamin A over a period of time may result in a condition called xerophthalmia (night blindness). By consuming foods rich in Vitamin A, like carrots, broccoli, tomatoes, liver, egg yolk, and butter, this condition can be reversed.
Our body needs oxygen for respiration. However, oxygen produces free radicals inside our body that can have a damaging effect on the retina of the eye. Many vitamins and minerals help in neutralizing these free radicals, and are, therefore, called antioxidants. Eating foods especially rich in vitamins A, C, and E can maintain a rich supply of antioxidants. Recent studies have indicated that antioxidants called ‘carotenoids’ present in coloured fruits and vegetables are very useful in maintaining eye health. Fresh fruits and vegetables can also help in preventing age-related degeneration of the macula (a part of the eye), thereby delaying age-related blindness.