Defects of Vision and their Correction

Question 1 What is myopia? State the two causes of myopia?

Question 2 With the help of ray diagram, show the eye defect myopia and correction of myopia using a lens?

Question 3 What is hypermetropia? State the two causes of hypermetropia?

Question 4 With the help of ray diagram, show the eye defect hypermetropia and correction of hypermetropia using a lens?

Question 5 What are the two common defects of vision?

Question 6 What type of spectacles should be worn by a person having the defects of myopia and hypermetropia?

Question 7 What is presbyopia? Write two causes of this defects? Name the type of lens which can be used to correct presbyopia?

Question 8 What is cataract? How is the vision of a person having cataract restored?

Question 9 What are the advantages of having two eyes instead of just one?

Question 10 What precautions should be taken while donating eyes?

Contents

• 1 Defects of vision and their correction
• 2 Myopia (Short-sightedness or Near-sightedness)
• 3 (1) Due to high converging power of eye-lens (because of its short focal length)
• 4 (2) Due to eye-ball being too long
• 5 Myopia (short-sightedness or near-sightedness) is corrected by using spectacles containing concave lenses.
• 6 Calculation of Power of Concave Lens to Correct Myopia
• 7 Hypermetropia (Long-sightedness or Far-sightedness)
• 8 (1) Due to low converging power of eye-lens (because of its large focal length)
• 9 (2) Due to eye-ball being too short
• 10 Hypermetropia (long-sightedness or far-sightedness) is corrected by using spectacles containing convex lenses
• 11 Calculation of the Power of Convex Lens to Correct Hypermetropia
• 12 Presbyopia
• 13 Cataract
• 14 Advantages of having Two Eyes

Defects of vision and their correction

The ability to see is called vision. It is also called eyesight. Sometimes the eye of a person cannot focus the image of an object on the retina properly. In such cases the vision of a person becomes blurred and he cannot see either the distant objects or nearby objects (or both) clearly and comfortably. The person is said to have a defect of vision. The defects of vision are also known as defects of eye.

There are three common defects of vision (or defects of eye). These are:

(1) Myopia (Short-sightedness or Near-sightedness)

(2) Hypermetropia (Long-sightedness or Far-sightedness), and

(3) Presbyopia.

These are the refractive defects of vision because they are caused by the incorrect refraction of light rays by the eye-lens. These defects of vision can be corrected by using suitable spherical lenses (convex lenses or concave lenses). These lenses are usually used in the form of eye-glasses or spectacles.

Myopia (Short-sightedness or Near-sightedness)

Myopia (or short-sightedness) is that defect of vision due to which a person cannot see the distant objects clearly (though he can see the nearby objects clearly). The far point of an eye suffering from myopia is less than infinity. Such a person can see clearly only up to a distance of few metres (or even less)

The defect of eye called myopia (or short-sightedness) is caused.

(1) Due to high converging power of eye-lens (because of its short focal length)

In an eye suffering from myopia, the ciliary muscles attached to the eye-lens do not relax sufficiently to make the eye-lens thinner to reduce its converging power. So, due to the greater converging power of the eye-lens in myopic eye, the image of a distant object is formed in front of the retina and hence the eye cannot see it clearly.

(2) Due to eye-ball being too long

In the eye suffering from myopia, the eye-ball is too long due to which the retina is at a larger distance from the eye-lens. This condition also results in the formation of the image of a distant object in front of the retina (even though the eye-lens may have correct converging power)

The parallel rays of light coming from the distant object O (at infinity) are converged to form an image in front of the retina due to which the eye cannot see the distant object clearly. The image is formed in front of the retina either due to high converging power of eye lens or due to eye ball being too long.

The far point of eye having myopia (or short-sightedness) is at point F which is less than infinity. The rays of light coming from the person’s far point F can just be focused by his eye on the retina. If the distant object can be made to appear as if it were at the far point F of this eye, then the eye can see it clearly. This is done by putting a concave lens in front of the eye.

Myopia (short-sightedness or near-sightedness) is corrected by using spectacles containing concave lenses.

The parallel rays of light coming from the distant object O (at infinity) are converged to form an image I in front of the retina due to which the eye cannot see the distant objects clearly.

The image is formed in front of the retina either due to high converging power of eye-lens or due to eye-ball being too long.

The far point of eye having myopia is at point F which is less than infinity. The rays of light coming from the person’s far point F can just be focussed by his eye on the retina. If the distant object can be made to appear as if it were at the far point F of this eye, then the eye can see it clearly. This is done by putting a concave lens in front of the eye.

Myopia is corrected by using spectacles containing concave lenses. When a concave lens L of suitable power is placed in front of the myopic eye. Then the parallel rays of light coming from the distant object are first diverged by the concave lens.

Due to this the concave lens forms a virtual image of the distant object at the far point F of this myopic eye. Since the rays of light now appear to be coming from the eye’s far point (F), they can be easily focused by the eye-lens to form an image on the retina. The concave lens used for correcting myopia should be of such a focal length (or power) that it produces a virtual image of the distant object (lying at infinity) at the far point of the myopic eye.

The concave lens used here decreases the converging power of the eye-lens and helps in forming the image of distant object on the retina of the myopic eye.

Calculation of Power of Concave Lens to Correct Myopia

The focal length of concave lens needed to correct myopia (or short-sightedness) in a person is calculated by using the lens formula:

The object distance u is to be taken as infinity (∞), and the image distance v will be the distance of person’s far point. Knowing the focal length of the concave lens, we can calculate its power.

Hypermetropia (Long-sightedness or Far-sightedness)

Hypermetropia (or long-sightedness) is that defect of vision due to which a person cannot see the nearby objects clearly (though he can see the distant objects clearly).

For example: A person having the defect hypermetropia cannot read a book clearly and comfortably though he can read the number of a distant bus clearly.

The near-point of a hypermetropic eye is more than 25 cm away. Such a person has to hold the reading material (like a book or newspaper) at an arm’s length, much beyond 25 cm from the eye for comfortable reading.

The defect of eye called hypermetropia (or long-sightedness) is caused

(1) Due to low converging power of eye-lens (because of its large focal length)

The ciliary muscles attached to the eye-lens become weak and cannot make the eye-lens thicker to increase its converging power. So, due to the low converging power of eye-lens in an eye suffering from hypermetropia, the image of nearby object is formed behind the retina and hence the eye cannot see it clearly.

(2) Due to eye-ball being too short

In an eye suffering from hypermetropia, the eye-ball is too short due to which the retina is at a smaller distance from the eye-lens. This condition also results in the formation of the image of a nearby object behind the retina.

The diverging rays of light coming from a nearby object O placed at the normal near point N are converged to form an image I behind the retina due to which the eye cannot see the nearby object clearly. The image is formed behind the retina either due to low converging power of eye-lens or because of eye-ball being too short.

Hypermetropia (long-sightedness or far-sightedness) is corrected by using spectacles containing convex lenses

When a convex lens (converging lens) L of suitable power is placed in front of the hypermetropic eye, then the diverging rays of light coming from the nearby object are first converged by this convex lens. Due to this, the convex lens forms a virtual image of the nearby object at the near point N’ of the hypermetropic eye.

The convex lens used for correcting hypermetropia (or long-sightedness) should be of such a focal length (or power) that it forms a virtual image of the object, at the near point N‘ of the hypermetropic eye.

The whole purpose of using a convex lens here is to increase the converging power of the eye-lens.

Calculation of the Power of Convex Lens to Correct Hypermetropia

The focal length of convex lens needed to correct hypermetropia (or long-sightedness) can be calculated by using the lens formula:

In this formula, the object distance u is to be taken as the normal near point of the eye and the image distance v will be the distance of the near point of the length of the convex lens, its power can be calculated.

Presbyopia

In old age, due to ciliary muscles becoming weak and the eye-lens becoming inflexible (or rigid), the eye loses its power of accommodation. Because of this an old person cannot see the nearby objects clearly.

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.

Presbyopia occurs in old age due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye-lens. The near-point of the old person having presbyopia gradually recedes and becomes much more than 25 centimetres away.

Presbyopia defect is corrected in the same way as hypermetropia by using spectacles having convex lenses.

A person suffering from myopia as well as hypermetropia uses spectacles having bifocal lenses in which upper part consists of a concave lens (to correct myopia) used for distant vision and the lower part consists of a convex lens (to correct hypermetropia) used for reading purposes.

It is possible to correct the refractive defects of the eye (such as myopia and hypermetropia) by using contact lenses or by undergoing surgical procedures.

Cataract

The medical condition in which the lens of the eye of a person becomes progressively cloudy resulting in blurred vision is called cataract. Cataract develops when the eye-lens of a person becomes cloudy (or even opaque) due to the formation of a membrane over it. Cataract decreases the vision of the eye gradually.

It can even lead to total loss of vision of the eye. The opaque lens is removed from the eye of the person by surgical operation and a new artificial lens is inserted in its place.

Advantages of having Two Eyes

Advantages of having two eyes are:

(1) Having Two Eyes Gives a Wider Field of View

A human being has a horizontal field of view of about 150° with one eye open but with two eyes open, the field of view becomes 180°. Thus, having two eyes gives a wider field of view. With two eyes open, we can see a much larger area in front of us.

Some animals, have their two eyes located on the opposite sides of their head. Those animals who have two eyes on the opposite sides of their head have the widest possible field of view. They can see much larger area around them than the human being can see. The animals of prey (like rabbit, deer, chicken, fish, etc.) have their two eyes on the opposite sides of their heads so that they can see their enemies (predators) in a very large area around them and try to escape from them.

(2) Having Two Eyes Enables Us to Judge Distances More Accurately

Our two eyes are a few centimetres apart from each other. Due to this, the two eyes see the same object from two slightly different angles and send two slightly different images of the same object to the brain. The brain combines these two slightly different images to build a three-dimensional picture of the object which enables us to judge the distance of the object more accurately.

Human beings (and all other animals having their eyes at the front of their head) are said to have stereoscopic vision (or stereopsis) which gives the perception of depth. All the predators (like tiger, lion, etc.) have their eyes at the front of head so that they can judge the distance of their prey accurately and catch them easily.

Some of the important points to be noted about the donation of eyes are the following:

(1) Any person of any age or sex (male or female) can donate eyes. People who wear spectacles or have undergone cataract operations can also donate eyes. Even the persons having ailments such as diabetes (sugar), hypertension (blood pressure) and asthma can donate eyes. But they should not have any communicable diseases.

(2) After the death of the person (who had registered for eye donation), the eye bank should be informed immediately. This is because eyes must be removed within 4 to 6 hours of a person’s death.

(3) The doctors of the eye bank team will remove the eyes at the home of the dead person or at the hospital. It takes only 10 to 15 minutes to remove the eyes.

(4) The eye bank distributes the donated eyes to various eye hospitals where these are transplanted in the blind people through surgical operation.

(5) Those donated eyes which are not suitable for transplantation, are used for doing research and for teaching purposes in medical colleges.