Defects of vision causes and correction

DEFECTS OF VISION: CAUSES AND CORRECTION

• Sometimes, eye gradually loss its power of accommodation.The vision becomes blurred due to the refractive defects of the eye.
• There are three common refractive defects of vision. These are:
  • Myopia (Short-sightedness or Near-sightedness)
  • Hypermetropia (Long-sightedness or Far-sightedness), and
  • Presbyopia.

MYOPIA (SHORT-SIGHTEDNESS OR NEAR-SIGHTEDNESS)

• Myopia is also known as near-sightedness or short sightedness.
• The images of distant objects are not formed at the right spots of rods and cones and hence appear blurred.
• A person with Myopia cannot see the distant objects clearly though he can see the nearby objects clearly.
• Far off objects cannot be seen as the eye lens does not produce a long focal length. However, nearby objects can be clearly seen.
• In a myopic eye, the parallel rays of light coming from the distant object(at infinity) are converged to form an image.
• The image is formed in front of the retina due to which the eye cannot see the distant object clearly.
• The light suffers a higher refraction than a normal eye and images of distant objects cannot be formed.

CAUSES OF MYOPIA:

It is caused due to:

• High converging power of eye-lens (because of its short focal length), or
• Eye-ball being too long or cornea bulged.
• Hereditary or due to uncontrolled diabetes or unattended cataract growths.

SYMPTOMS OF MYOPIA:

• Squinting
• Frowning
• Tired eyes
• Severe headache due to overstrained eyes.
• Also, people suffering from myopia face difficulty in reading from a specific distance.

HOW TO CORRECT MYOPIA?

Myopic eye do not diverge light rays coming from far off objects and hence a focused image cannot be formed on the retina. In fact, the rays converge much before they reach the retina. So, myopia is corrected by spectacles containing concave or diverging lenses of suitable focal length which will bring the image back on to the retina.

Focal length of the correcting lens = Distance of the far point from the eye.

HYPERMETROPIA (LONG-SIGHTEDNESS OR FAR-SIGHTEDNESS)

• Hypermetropia is also known as long-sightedness or far-sightedness.
• A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly.
• The images of nearby objects are formed behind the retina.
• The light rays from nearby objects do not focus on the retina surface creating a blurry image.
• Distant objects can be clearly seen.
• The near-point of a hypermetropic eye is more than 25 centimeters away which is the normal near point of the human eye.

CAUSES OF HYPERMETROPIA:

It is caused due to:

• Low converging or focussing power of crystalline eye-lens (because of its large focal length), or
• Eye-ball being too short.
• Hypermetropias are sometimes present at the birth of children but as they grow older, the eyeball lengthens to normal and the defect is cured.

HOW TO CORRECT HYPERMETROPIA?

• Hypermetropia is corrected by using spectacles containing convex lenses of suitable power.
• $Focal Length of correcting lens =\frac{yD}{y–D}$

  Where y = Distance of the near point from the defective eye.

• However, instead of using eyeglasses, contact lenses are more preferable and safe to use and has the same effect.
• Another way to correct this defect is to alter the refractive power by surgery like Lasik surgery.
• Hypermetropic eye has positive power like or as corrective powers. This indicates that the corrective lens required is convex. Such lenses are mostly used during reading or using laptops, etc.

PRESBYOPIA

• Presbyopia is that defect of vision due to which an old person cannot see the nearby objects clearly due to loss of power of accommodation of the eye.
• The near-point of the old person having presbyopia gradually recedes and becomes much more than 25 centimeters away.

CAUSES OF PRESBYOPIA:

It is caused due to:

• Gradual weakening of the ciliary muscles and
• Diminishing flexibility or stiffness of the eye lens’
• Both these occur with the natural aging of the eye between 40 to 60 years of age, although the person may have had normal vision throughout their life.

HOW TO CORRECT PRESBYOPIA?

• Presbyopia defect is corrected by using bi-focal lenses which 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. Such difference is not visible in progressive lenses which also function in the same way but does not have a line distinguishing the two lenses. Since the transition is smoother it is more comfortable.
• Presbyopia may also be corrected with eyeglasses or contact lenses having convex lens, if bot myopia and hypermetropia are not present simultaneously.
• People having both defects also use monovision.
• Contact lenses used have each lens correcting one defect. However, the effective perceptive may not be very good.
• Modified monovision contact lenses solve problem of depth perception.

CATARACT

• During old age, the crystalline lens of people becomes milky and cloudy. This causes partial or complete loss of vision.
• The vision is restored through a cataract surgery.

ASTIGMATISM

• Light refracts or bends evenly when it enters a perfectly spherical lens, such as the eye. But, if the lens is not completely spherical like the back of a spoon, the light rays may be refracted unevenly in multiple directions. So, the image of the object appears blurred. Such an optical defect is called astigmatism.
• If an astigmatic person looks at the spokes of a wheel, some of them appear clearer than others, because only parts of the image are correctly focused. Astigmatism is one of a group of eye conditions known as refractive errors. Distortion is caused at all distances.
• Refractive errors disturb the way such that light rays are focused within the eye.

Causes of Astigmatism

• Irregularly shaped cornea
• Distorted lens

Types of Astigmatism

1. Corneal astigmatism: This is due to irregular shape of cornea.
2. Lenticular astigmatism: This is due to distorted shape of lens.

COLOUR BLINDNESS

A person who cannot distinguish between various colours but can see well otherwise, is said to be colour-blind.

Causes of colour blindness

Colour blindness is caused due to lack of some cones in the retina of the eyes.