Human Eye & Eye Defects

Our eyeball is nearly spherical with white outer layer called the sclera. Here is a short description of how our eye works.

Working of Human Eye

The light enters the eye through a curved transparent tissue called Cornea. In humans, the refractive power of the cornea is approximately 43 dioptres. While the cornea contributes most of the eye’s focusing power, its focus is fixed. The curvature of the lens, on the other hand, can be adjusted to “tune” the focus depending upon the object’s distance. The cornea has no blood supply; it gets oxygen directly through the air. Oxygen first dissolves in the tears and then diffuses throughout the cornea to keep it healthy.

Behind the cornea, is a circular diaphragm called iris which has a central hole called pupil. The size of the pupil aperture is adjusted by muscle action and controls the amount of light entering the eye. The converging crystalline lens composed of glassy fibres is situated behind the iris. The shape and curvature of the crystalline lens is controlled by ciliary muscles. The images are formed on the retina by adjusting and changing the curvature of the lens. This is called accommodation of the eye. The eye ball contains a fluid in front of the lens and a gelatinous material in the space behind it. The retina of the eye consists of two types of photo sensitive rods and cones. The more numerous rods have a greater sensitivity to light, but do not respond to colour. Rods work well when the light is dim. The cones are sensitive to bright light and colour. They are helpful to us to see things in colour. Special optical nerves carry the messages from retina to the brain which interprets the images as erect images. The functioning of the eye is similar in many ways to that of a camera. Both have a lens but eyes are advanced because curvature of the camera lens cannot be changed.

The points between which the eye can see distinctly are called far point and near point. The far point is normally without limit (infinity) and near point depends on the accommodation of the crystalline lens.

When we move a pencil slowly towards our nose. At some points, we observe that the pencil appears blurred. This is the near point. For a normal human eye, the near point is 25 cm from the eye.

The near point increases with the age as shown below:

Short-sightedness or Myopia

The inability to see the distant objects clearly and distinctly is called short sightedness. This defect arises when the image is formed infront of the retina. A short sighted person can see near objects clearly. This may arise due to either excessive curvature of the cornea or elongation of the eyeball. This defect is corrected by wearing glasses with a concave lens.

Long sightedness (or) Hypermetropia

The inability to see near objects clearly and distinctly is called long sightedness. This defect arises when the image is formed behind the retina. This defect may arise due to shortening of eye ball. A long sighted person can see the distant objects clearly. This defect is corrected by wearing spectacles with convex lens (converging) of appropriate focal length. A converging lens will correct this defect by converging the incoming rays so that the image is formed on the retina.

Longsightedness occurs naturally with age. As we grow old, our ciliary muscles weaken, and the crystalline lens looses its elasticity or hardens which limits the eye’s accommodation. Some persons may have both Longsightedness and short sightedness defects. They should wear glasses consisting of both converging and diverging lenses on the same piece of glass. This is called bifocal glass. The bifocal lens was invented by Benjamin Franklin.