The eye has a nearly spherical shape of diameter 1 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, space is filled with a liquid called the aqueous humor and behind that a crystalline lens.
Between the aqueous humor and the lens. we have a muscular diaphragm called iris, which has a small hole in it called pupil. The amount of light entering the eye is controlled by varying the aperture of the pupil with the help of the iris.
The lens is hard at the middle while gradually 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. The retina contains about 125 million receptors called rods and cones which receive the light signal and about one million optic nerves which transmit the information to the brain.
The space between the lens and retina is filled with another liquid called the vitreous liquid. The refractive index for aqueous humor and vitreous liquid are same i.e 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 bending occurs at 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.
The cornea – lens – fluid system is equivalent to a single converging lens whose focal length may be adjusted by the ciliary muscles.
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.
When the eye is focussed on a closer object, the ciliary muscles are strained and the focal length of the eye – lenses 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 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 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 muscle cannot sustain a large strain and the near point shifts to large values, say, 1 to 2 m or even more. The average value of the least distance for clear vision for a normal eye is generally taken to be 25 cm.
APPARENT SIZE OF AN EYE
The size of an object as sensed by us is related to the size of its image formed on the retina. A larger image on the retina activates larger number of rods and cones attached to it and if an object is taken away from the eye, the size of the image on the retina decreases and hence, the same object looks smaller. It is also clear from figure that the size of the image on the retina is roughly proportional to the angle subtended by the object on the eye. This angle is known as the visual angle and optical instruments are used to increase this angle artificially in order to improve the clarity.