Human Eye


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:

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:

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.

Presbyopia

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.

Twinkling of stars:

There twinkling of a star as due to atmospheric refraction of starlight. The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The star appears slightly higher (above) than its actual position when viewed near the horizon. Further, this apparent position of the star is not stationary, but keeps on changing slightly, since the physical condition of the earth’s atmosphere are not stationary, as was the case in the previous paragraph. Since the stars are very distant, they approximate point-size sources of light. As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers- the star sometimes appears brighter and at some other time, fainter which is the twinkling effect.

Tyndall Effect:

The earth’s atmosphere is a heterogeneous mixture of minute particles. These particles include smoke, tiny water droplets, suspended particles of dust and molecules of air. When a beam of light strikes such fine particles, the Path of the beam becomes visible. The light reaches us, after being reflected diffusely by these particles. The phenomenon of scattering if light by the colloidal particles gives rise to Tyndall effect which you have studied in class 9th . This phenomenon is seen when a fine beam of sunlight enters a smoke-filled room through a small hole. Thus, scattering of light makes the particles visible. Tyndall effect can also be observed when sunlight passes through a canopy of a dense forest. Here, tiny water droplets in the mist scatter light.         The colour of the scattered light depends on the size of the scattering particles. Very fine particles scatter mainly blue light while particles of larger size scatter light of longer wavelengths. If the size of the scattering particles is large enough then, the scattered light may even appear white.

Why is the colour of the clear Sky Blue?

The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the rd end. The red light has a wavelength about 1.8 times greater than blue light. Thus, when sunlight passes through the atmosphere, the fine particles in air scatter blue colour(shorter wavelengths) more strongly than red. The scattered blue light enters our eyes. If the earth had no atmosphere, there would not have been any scattering. Then the sky would have looked dark. The sky appears dark to passengers flying at very high altitudes s scattering is not prominent at such heights.