Defects in Human Eyes

The human eye is an interesting system that helps in turning light rays into images. The light gets reflected by objects that first hit the cornea, which is protected via a clear tissue covering known as the epithelium. This cornea is responsible for approximately 70% of the total light refraction or focusing of the light.

The light then travels through the pupil, which is the central opening of the iris, and then it  reaches the lens. The lens is made for doing the final focus adjustment so that the light gets focused, after it reaches the retina. At the retina, the light rays are translated into electrical impulses and then they are sent to the optic nerve. The optic nerve carries these impulses up to the brain, which further translates them into images.

Working of the Eye

The light rays coming from any object enter the eyes via the pupil and fall on the eye lens. The eye lens then helps in converging the light rays to produce an image of the object on the retina that is real and inverted. Retina possesses a large number of light-sensitive cells which have the ability to generate electrical signals. After the image formation on the retina it sends electrical signals to the brain and the person gets a sensation of image. Even though the image formed on the retina is inverted, our brain interprets it as erect. Therefore, the eye lens is the convex lens and the retina represents the screen of the eye.

Defects in Human Eyes

Visual Acuity

Visual ability is defined as visual acuity in medical circles. “Normal” vision relates to an acuity of 1.0 ( mainly 6/6 in metric countries). A person possessing this acuity will be able to see a 1.5 mm object at five metres distance. A person who cannot see objects smaller than 3 mm at this distance usually possess an acuity of 0.5 (i.e. 6/12). Majority of the people possess better than normal vision; young people quite often have visual acuity ranging between 1.0 and 2.0 (6/6 – 6/3). This gradually decreases with age and could become even worse than 1.0. 

Various people could have better vision but they may not be aware of it. What in medical terms is considered as “normal” vision is not in fact all that normal, as 48% of people in Europe wear glasses, excluding the ones with contact lenses and people with undiagnosed or untreated visual defects. The vast majority of these defects are mainly due to a mismatch between corneal shape and eye length. These are known as the “refractive errors,” i.e. problems in the refraction of the light by the cornea, so that the light does not get focused on the retina. 

Myopia or Near-Sightedness

It is a defect of vision whereas far-off objects seem to be blurred and objects near can be seen clearly. As the eyeball is too long or the refractive power of the eye lens is too high; the image gets formed in front of the retina rather than forming on it. Correction of myopia can be done via wearing glasses/contacts made of concave lenses in order to help focus the image formed on the retina. Both contact lenses and glasses help in correcting myopia via diverging the incident light rays in the eyes so that the eye’s focal point falls onto the retina. 

Hypermetropia or Long sightedness

Hypermetropia generally is a defect of vision in which there is a difficulty in viewing objects that are near but the same person can view far objects easily. As because the eyeball is too short or the eye lens’s refractive power is very weak thus the image instead of being forming upon the retina, they are formed behind the retina. Correction of hypermetropia can be done via wearing glasses/contacts consisting of convex lenses.

Cataract

It represents the clouding in the lens, which prevents the formation of a clear, sharp image. A cataract is usually formed when old cells after they die, stick in a capsule, with the passing of time a clouding gets formed over the lens. Due to this clouding gets formed and blurred images are also formed. Correction of cataract done via a surgery. An artificial lens in place of the opaque lens is placed after removing it via surgery.

Presbyopia or Old-age Long sightedness

It is a natural defect that occurs with age. In presbyopia, the ciliary muscles get weak and are no longer able to adjust the eye lens. The eye muscles have  become so weak that no longer can a  person be able to see nearby objects clearly. The near point of a person suffering with presbyopia is mainly more than 25cm.  Correction of presbyopia can be done after wearing bifocal glasses or Progressive Addition Lenses (PALs) whereas the upper region of the lens comprises concave lens and lower portion is composed of a convex lens. A person with presbyopia also suffers just myopia or just hypermetropia.

Astigmatism

It is a defect in which the light rays after entering the eye do not help in focusing the light evenly to a single focal point on the retina but instead they scatter away. The light rays in a way get some focus on the retina and some focus in front of or behind it. This happens due to the non-uniform curvature of the cornea; that results in a distorted or blurry vision at any distance. Astigmatism can be corrected by using a special spherical cylindrical lens.

Cataract

It is the clouding of the lens that prevents the formation of a clear, sharp image. A cataract is caused as the lens is sealed in a capsule and as the old cells die they get trapped in the capsule, with time this leads to the formation of clouding over the lens. This clouding leads to the formation of blurred images.

Glaucoma

The eye releases a clear fluid (known as the aqueous humor) which fills the space between the cornea and the iris. This fluid is filtered out via a complex drainage system. It represents a balance between the production and drainage of this fluid that helps in determining the eye’s intraocular pressure (IOP). Glaucoma is a disease caused by an increased IOP generally resulting from a malfunction in the eye’s drainage system. Increased IOP can lead to irreversible damage of the optic nerve and retinal fibers and if they are left untreated they can result in a permanent loss of vision.

Power of Accommodation

Power of accommodation represents the process in which ciliary muscles function, to adjust the focal length of the eyes in order that clear image gets formed on the retina. This varies greatly for far or nearby objects. For a normal eyesight, the power of accommodation is usually calculated to be 4 dioptre.

Conclusion

The Human eye is a very delicate organ, but it possesses an advanced mechanism in which the human eye and its organs work together with the nerves of the human mind in order to help in the process of image formation. Proper care should be taken of the eyes as it is the most important sense which is necessary for our daily functioning. Here we come to an end of this topic. We hope that you were able to grasp a clear concept of topics like defects in human beings.