The human eye is one of the most valuable and sensitive sense organs. It enables us to see the wonderful world and the colors around us. On closing the eyes, we can identify objects to some extent by their smell, taste, sound they make or by touch. It is, however, impossible to identify colors while closing the eyes. Thus, of all the sense organs, the human eye is the most significant one as it enables us to see the beautiful, colorful world. The human eye is like a camera. Its lens system forms an image on a light-sensitive screen called the retina. Light enters the eye through a thin membrane called the cornea. It forms the transparent bulge on the front surface of the eyeball. The eyeball is approximately spherical in shape with a diameter of about 2.3 cm. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea. We find a structure called iris behind the cornea. Iris is a dark muscular diaphragm that controls the size of the pupil. The pupil regulates and controls the amount of light entering the eye The crystalline lens merely provides the finer adjustment of focal length required to focus objects at different distance on the retina. The eye lens forms an inverted real image of the object on the retina. The retina is a delicate membrane having enormous number of light-sensitive cells. The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive object as they are.
Power of accommodation:
They eye lens is composed of a fibrous, jelly-like material. Its curvature can be modified to some extent by the ciliary muscles. They change in the curvature of the eye lens can thus change its focal length. When the muscles are relaxed, the lens becomes thin. Thus its focal length increases. This enables us to see distant objects clearly. When you are looking at objects closer to the eye, the ciliary muscles contract. This increases the curvature of the eye lens. The eye lens then becomes thicker. Consequently the focus length of the eye lens decreases. This enables us to nearby objects clearly.
The ability of the eye lens to adjust its focus length is called its power of accommodation.
However, the focus length of the eye lens cannot be decreased below a certain minimum limit. The minimum distance, at which objects can be seen most distinctly without strain, is called the least distance of distinct vision. It is also called the near point of the eyes. For a young adult with normal vision, the near point is about 25 cm. The farthest point up to which the eye can see objects clearly is called the far point of the eye. It is infinity for normal eye.
DEFECTS OF VISION AND THEIR CORRECTION:
Sometimes, the eye may gradually lose its power of accommodation. In such condition, the person cannot see objects distinctly and comfortably. There are mainly three common refractive defects of vision. These are (1) Myopia or near-sightedness, (2) Hypermetropia or far-sightedness, and (3) Presbyopia. These defects can be corrected by the use of suitable spherical lenses. We discuss below these defects and their correction.
Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself. This defect may arise due to (1) excessive curvature of the eye lens, or (2) elongation of the eyeball. This defect can be corrected by using a concave lens of suitable power. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
Hypermetropia is also known as far-sightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. The near point, for the person, is farther away from the normal near point (25cm). This is because the light rays are focussed at a point behind the retina. This defect arises either because (1) the focal length of the eye lens is too long, or (2) the eyeball has become too small. This defect can be corrected by using convex lens of appropriate power.
Sometimes, a person may suffer from both myopia and hypermetropia and this defect is known as presbyopia. This happens because power of accommodation of the eye usually decreases with ageing. For most people, the near point gradually recedes away. They find it difficult to see nearby as well as far objects comfortably and distinctly. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens. This defect is cured by using a bi-focal lenses. A common type of bi-focal lenses consists of both concave and convex lenses. The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision.