The Human Eye and The Colorful World


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Introduction

  • The human eye is one of the most valuable and sensitive sense organs. It empowers us to see the wonderful and colorful things around us.

Human Eye

  • The human eye is very much similar to a camera.

  • The lens system of an eye forms an image on a light-sensitive screen known as the retina (see the image given below).

  • Light enters the eye through a thin membrane known as the cornea.

Eye
  • With a diameter of about 2.3 cm, the eyeball is almost spherical in shape.

  • Behind the cornea, there is a structure, known as iris (see the image given above).

  • Iris is a dark muscular diaphragm; it has major function to control the size of the pupil.

  • The pupil has major function to control and regulate the amount of light entering the eye.

  • The lens of eye forms an inverted real image of an object on the retina.

Power of Accommodation

  • The eye lens, which is composed of a fibrous, jelly-like material, has an important role; further, the change in the curvature of the eye lens also change the focal length.

  • When the muscles are in relax state, the lens shrinks and becomes thin; therefore, its focal length increases in this position and enables us to see the distant objects clearly.

  • On the other hand, when you look at an object closer to your eye, the ciliary muscles contract; resultantly, the curvature of the eye lens increases and the eye lens becomes thicker. In such a condition, the focal length of the eye lens decreases, which enables us to see nearby objects clearly.

  • Such ability of the eye lens to adjust its focal length is known as accommodation.

  • Furthermore, the focal length of the eye lens cannot be decreased below a certain (minimum) limit. This is the reason that we cannot read the book kept very close to our eyes rather we have to maintain a certain distance.

  • To see an object comfortably and clearly, the object must be kept at the distance of (about) 25 cm from the eyes.

  • However, there is no limit of the farthest point; a human eye can see objects of infinity, e.g. moon, stars, etc.

Defects of Vision and Their Correction

  • When the crystalline lens of an eye (normally at old age), becomes milky and cloudy, it is known as cataract.

  • Cataract causes partial or complete loss of vision; however, it can be treated through a cataract surgery.

  • Following are the three common refractive defects of vision −

    • Myopia or near-sightedness

    • Hypermetropia or farsightedness, and

    • Presbyopia

  • Let’s discuss each them in brief:

Myopia

  • Myopia is also known as near-sightedness.

  • A person suffering from myopia can see nearby objects clearly, but cannot see distant objects clearly.

Myopia
  • As shown in the image given above, in a myopic eye, the image of a distant object is formed in front of the retina instead of at the retina.

  • Myopia may arise because of −

    • Excessive curvature of the eye lens, or

    • Elongation of the eyeball.

  • This defect can be corrected by using a concave lens of suitable power.

  • As shown in the image given above, using of concave lens of suitable power, brings the image back on to the retina; likewise, the defect is corrected.

Hypermetropia

  • Hypermetropia is also known as far-sightedness.

  • A person suffering from hypermetropia can see the distant objects clearly, but cannot see the nearby objects clearly.

  • In such a case, as shown in the image given below, the near point, is farther away from the normal near point (i.e. 25 cm).

Hypermetropia
  • Hypermetropia may arise because of

  • The focal length of the eye lens – when it is too long, or

  • The eyeball has become too small.

  • Hypermetropia can be corrected by using a convex lens of appropriate power.

  • As shown in the image given above, eye-glasses with converging lenses provide the additional focusing power that helps in forming the image on the retina.

Presbyopia

  • Presbyopia, an eye problem, arises because of the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.

  • Some people suffer from both myopia and hypermetropia; such kind of eye defect is treated by using bifocal lenses.

  • A common type of bi-focal lenses consists of both the concave and the convex lenses.



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