Human Eye and Defects of Vision

Every day you ‘see’ beautiful things around you. You don’t just ‘see’ things you observe them, analyze them and then you either keep staring or choose to ignore. What enables us to look at things around us? What makes human eye so wonderful and essential? Yet, if on one hand, our eyes enable us to have a look at the world around us; it also carries some defects. Let us know more about the human eye and its defects below.

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The Human Eye and Its Structure

‘Human Eye’ is the organ of vision of the human body that enables us to see.  The human eye(s) are located in the specialized sockets carved out in the human skull. Each human eye sizes for approximately 2.5 cm in diameter. The main parts of a human eye are:

Sclera

The white outer region of our eye which protects the internal parts of our eyes is known as ‘sclera‘. Its made up of fibrous tissues. It’s continuous with the cornea.

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Cornea

The transparent portion of our eye that allows the light to enter our eye is known as the ‘cornea’ and is made up of transparent tissue. The cornea covers the pupil, anterior chamber, and the iris. Along with the anterior chamber and lens, cornea refracts light and accounts for two-thirds of the eye’s total power. Usually, the refractive power of the cornea is approximately 43 dioptres.

Iris

A circular, thin structure made up of contracting and relaxing muscles in the eye that controls the size of the pupil and the light reaching the retina are known as the ‘iris’. Iris defines a person’s eye color. If we define the human eye as a camera then the iris becomes aperture of the eye.

 

Pupil

The part of the eye located in the center of the iris allowing light to reach the retina. The pupil appears black in color since the eye tissues absorb or diffusely reflect the light entering the pupil. Iris controls the pupil.

Lens

The lens is a biconvex, transparent structure present in the eye behind the pupil. The lens along with the cornea refracts the light, so as to focus it on the retina. By changing its shape, the lens is capable of changing the focal distance of the eye. 

Retina

The retina is a light-sensitive tissue in the inner coat of the eye that sends electrical signals after converting them from light to the brain for processing. Rods and cones are the two light-sensitive types of cells present in the retina. Rods help us for night-time vision and cones help us see colors. 

Optic Nerve

The optic nerve sends electrical impulses from the retina, at the back of the eyes to the brain.

Defects of The Eye and Their Corrections

As perfect the human eye may seem; it’s not. And if the human eye isn’t perfect, which means it has its share of defects of the human eye. Here are few common defects of the human eye:

a. Myopia or Near-Sightedness

Myopia is a defect of vision wherein far-off objects appear blurred and objects near are seen clearly. Since the eyeball is too long or the eye lens’s refractive power is too high; the image forms in front of the retina rather than forming on it. Correction of myopia can happen by wearing glasses/contacts made of concave lenses to help focus the image on the retina.

 

b. Hypermetropia or Longsightedness

Hypermetropia is a defect of vision wherein there is difficulty in viewing objects that are near but one can view far objects easily. Since the eyeball is too short or eye lens’s refractive power is too weak hence the image instead is of being forming upon the retina, its forms behind the retina. Correction of hypermetropia can happen by wearing glasses/contacts containing convex lenses.

Source: Optometriceyesitenc.com

c. Cataract

Cataract is the clouding of the lens, that prevents the formation of a clear, sharp image. A cataract forms when old cells after they die, stick in a capsule wherein with time a clouding over lens happens. Because of this clouding blurred images are formed. Correction of cataract can happen through a surgery. An artificial lens in place of the opaque lens is after removing it via surgery.

d. Presbyopia or Old-age Longsightedness

Presbyopia is a natural defect that occurs with the age. In presbyopia, the ciliary muscles become weak and are no longer able to adjust the eye lens. The eye muscles become so weak that no longer can a person see nearby objects clearly. The near point of a person with presbyopia is more than 25cm.  Correction of presbyopia can happen by wearing bifocal glasses or Progressive Addition Lenses (PALs) wherein the upper portion of the lens contains concave lens and lower portion contains a convex lens.
A person with presbyopia can also have just myopia or just hypermetropia.

e. Astigmatism

Astigmatism is a defect wherein the light rays entering the eye do not focus light evenly to a single focal point on the retina but instead scatter away. The light rays in a way where some focus on the retina and some focus in front of or behind it. This happens because of non-uniform curvature of the cornea; resulting in a distorted or blurry vision at any distance. Correction of astigmatism can happen by using a special spherical cylindrical lens.

Power of Accommodation

Power of accommodation is the process by which ciliary muscles function, to adjust the focal length of the eyes so that clear image forms on the retina. This varies far or nearby objects. For a normal eyesight, the power of accommodation is 4 dioptre.

You can download Human Eye and the Colorful World Cheat Sheet by clicking on the download button below

 

Solved Example for You

Q. If a person cannot see an object clearly when it is placed at about 25 cm away from him, he is suffering from:

a. Myopia

b. Presbyopia

c. Hypermetropia

d. Astigmatism

e. None of these

Sol: c. Hypermetropia

Hypermetropia is a defect in which a person is unable to see close objects clearly. This happens because of shortening of eyeball or loss of power of eye lens resulting in an increase of the near point.