Why do people need glasses?

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Glasses are associated with a variety of personality traits – it is a common notion that intelligent people have glasses, obtained due to long hours of reading or studying. The real reason, however, is not remotely related to the personality or the intelligence quotient of a person. It is related to the defect in the eyes of the person.

Our eyes are eerily similar to a LENS. A lens made up of glass or transparent material with curved edges, capable of dispersing light. Like, a lens, our eyes can also disperse light. A lens has the following properties

  1. The PRINCIPLE AXIS, an axis perpendicular to the placement of a lens.
  2. The FOCAL POINT, the point on the principal axis where all the light rays cross.
  3. The FOCAL LENGTH, the length from the focal point to the center of the lens.
  4. The POWER, which is the reciprocal of the focal length.
lens
lens vision

A RAY DIAGRAM is a diagram depicting the formation of an image after passing through the lens. Lenses are of two types and their ray diagrams are constructed for the type of lens used – concave or convex. Our eye lens is a CONVEX LENS, with its focal point being the RETINA. Thus, the light rays entering the eye (i.e., the eye lens) fall on the retina, which sends the brain a message of the image formed in front of it.

lens converging

The eye is excellent at regulating the image formed by adjusting the amount of light entering the eye. In bright places, we squint our eyes, enabling less light to enter and in dark places, our eyes dilate to enable entry of more light – both performed to facilitate the formation of a clear image.

The LEAST DISTANCE OF DISTINCT VISION of a human eye is the shortest length (from the image to the eye) which can form a clear, distinct image. This value is approximately 25cm – meaning the human eye will see an object closer than 25cm to the eye will appear blurry. In case the least distance of distinct vision is less than 25cm, the person is said to suffer from HYPEROPIA.

 

What causes hyperopia?

Hyperopia is a condition in which objects nearby may not be visible, while those far away will be relatively distinguishable. It is also known as FARSIGHTEDNESS. In hyperopia, the image is not formed at the retina, but misses it and falls behind it. Hence, the object nearby appears blurry when the signal is sent to the brain.

Hyperopia is caused due to exposure to bright light at proximity to the eye or reading a book or any close-by object in the dark. It can also be acquired in old age due to the inevitable ageing of the eye lens.

It is corrected using PLUS-POWERED LENS or the CONVEX lens, which diverge the rays of light in such a way that they fall on the retina and the subsequent image formed is clear. The extent of hyperopia is checked by ophthalmologists and technicians using a standard eye test protocol – and a corresponding power (reciprocal of the focal length of the lens) is suggested to correct the defect.

hyperopia vision

What causes myopia?

The opposite of hyperopia is myopia, a condition in which the light rays entering the eye form an image before (or ahead) of the retina. Hence, the resultant image formed appears blurry. Myopia is generally called NEARSIGHTEDNESS, wherein objects nearby can be visualized clearly, but those placed far do not appear clear.

Myopia is caused due to elongated exposure to screens of mobile phones, computers, etc. A person suffering from myopia exhibits a habit called a PINHOLE CAMERA EFFECT – which refers to squinting the eyes to reduce the size of the light entering the eye, which may result in a clearer image. This is the symptom that an eye test is due.

It is corrected using NEGATIVE-POWERED LENS or CONCAVE LENS, which helps in ensuring that the image is formed on the surface of the retina instead of in front of it. Like in the case of hyperopia, the extend of myopia determines the power of the corrective lens used.

While myopia and hyperopia are the most common types of refractive defects observed, there are other defects such as astigmatism.

myopia vision

What is astigmatism?

Astigmatism is a defect in the curvature of the eye lens or due to the irregular shape of the CORNEA. It can be inherited and present from birth. Due to the abnormal shape of the eye lens, the image can fall out of focus at random and cause blurry vision.

Astigmatism is corrected using special lenses called PROGRESSIVE LENS, which have different powers in different regions of the lens. People with PRESBYOPIA, a condition with myopia as well as hyperopia are also prescribed to use progressive or bifocal lenses.

cornia iris

Many households claim to have homemade remedies to get rid of such defects, but these do not have a scientific backbone. On the other hand, while glasses are prescribed as being ‘corrective measures’, they do not ensure that the defect present will get cured. While in some cases, glasses can inhibit the increase of the defect, the permanent solution for this remains LASIK surgery or laser eye surgery is performed.

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