The Human Eye Parts Construction And Mechanism

THE HUMAN EYE PARTS CONSTRUCTION AND MECHANISM

THE EYE

Optical instrument are used primarily to assist the eye in viewing an object. Let us first discuss in brief the construction of a human eye and the mechanism by which we see, the most common but most important experiment we do from the day we open our eyes.

Figure shows schematically the basic components of diameter about at inch. The front portion is more sharply curved and is covered by a transparent protective membrane called the cornea. It is this portion which is visible from outside. Behind the cornea, we have a space filled with a liquid called the aqueous humor and behind that a crystalline lens.

PARTS

Iris And Pupil

Between the aqueous humor and the lens, we have a muscular diaphragm called iris, which has a small hole in it called pupil. Iris is the coloured part that we see in an eye. The pupil appears black because any light falling on it goes into the eye and there is almost no chance of light coming back to the outside. The amount of light entering the eye, may be controlled by varying the aperture of the pupil with the help of the iris. In low-light condition, the iris expands the pupil to allow more light to go in. In good light conditions, it contracts the pupil.

Lens 

The lens is hard in the middle and gradually becomes soft towards the outer edge. The curvature of the lens may be altered by the ciliary muscles to which it is attached. The light entering the eye forms an image on the retina which covers the inside of the rear part of the eyeball. The retina contains about 125 million receptors called rods and  cones which receive the light signal and about one million optic-nerve fibres which transmit the information to the brain. The space between the lens and the retina is filled with another liquid called the vitreous humor.

Aqueous Humor

The aqueous humor and the vitreous humor have almost same refractive index 1.336. The refractive index of the material of the lens is different in different portions but on the average it is about 1.396. When light enters the eye from air, most of the behind occurs at the cornea itself because there is a sharp change in the refractive index. Some additional bending is done by the lens which is surrounded by a fluid of somewhat lower refractive index. In normal conditions, the light should be focussed on the retina.

Cornea Lens Fluid

The cornea-lens-fluid system is equivalent to a single converging lens whose focal length may be adjusted by the ciliary muscles. Now onwards, we shall use the word eye-lens to mean this equivalent lens.

When the eye is focussed on a distant object, the ciliary muscles are relaxed so that the focal length of the eye-lens has its maximum value which is equal to its distance from the retina. The parallel rays coming into the eye are then focussed on the retina and we see the object clearly.

Ciliary Muscles

When the eye is focussed on a closer object, the ciliary muscles are strained and the focal lenght of the eye-lens decreases. The ciliary muscles adjust the focal length in such a way that the image is again formed on the retina and we see the object clearly. This process of adjusting focal length is called is called accommodation. However, the muscles cannot be strained beyond a limit and hence, if the object is brought too close to the eye, the focal length cannot be adjusted to form the image on the retina. Thus, there is a minimum distance for the clear vision of an object.

The nearest point for which the image can be focussed on the retina, is called the near point  of the eye. The distance of the near point from the eye is called the least distance for clear vision. This varies from person to person and with age. At a young age (say below 10 years), the muscles are strong and flexible and can bear more strain. The near point may be as close as 7-8 cm at this age. In old age, the muscles cannot sustain a large strain and the near point shifts to large values, say 1 to 2 m or even more. We shall discuss about these defects of vision and use of glasses in a later section. The average value of the least distance for clear vision for a normal eye is generally taken to be 25cm.