Human Eye

One of the most complicated optical devices is the human eye. Let us see the construction of the human eye and then the mechanism of image formation.

Human eye is spherical in shape and diameter of about 2.5cm. Sclerotic is a tough, opaque and white substance forming the outermost coating of the yet. The front portion is sharply curved and covered by a transparent protective membrane called the 'cornea'. Inner to the sclerotic there is a layer of black tissue called as choroids consisting of a mass of blood vessels, which nourishes the eye. The black color does not reflect the light and hence rules out the blurring of image by reflection within the eyeball.

Behind the cornea, the space is filled with a liquid called the aqueous humor and behind that a crystalline lens. 'Iris' is a muscular diaphragm lying between the aqueous humor and the crystalline lens. Iris has an adjustable opening in the middle called the pupil of the eye. The pupil appears black because all the light entering is absorbed by the 'retina', which covers the inside of the rear part of the ball. Iris controls the amount of light emerges because the retina absorbs nearly all the light, which falls upon it. This is done by varying the aperture of the pupil with the help of the iris. In dim light the iris dilates the pupil and so that more light can enter in. when the light is bright the pupil contracts.

The crystalline lens divides the eyeball into two chambers. The chamber between the cornea and the lens is called the anterior chamber filled with a fluid called aqueous humour while the chamber between the lens and the retina is called the posterior chamber which is filled with a transparent gelatinous substance called vitreous humour.

The refractive indices of the cornea, pupil lens and fluid portion of the eye are quite similar. So, when a ray of light enters the eye it is refracted at the cornea. This refraction produce a real inverted and diminished image of distant objects on the retina.

When the object is kept at different distances then we may expect the image to be formed at different distances from the lens. It means it may not form on the retina always. But in reality it is not so. Image is always formed on the retina. This is possible because the curvature of the crystalline lens is altered by ciliary muscles. When the eye is focused on infinity the muscles are relaxed and the eye lens remains thin. If the object is brought nearby the curvature increases so that the image can be formed on the retina. This property of the eye lens is called as accommodation.

Then the question comes that how long does the image persist on the retina. It is as long as the eye focuses the object or does it longer on after that also? It is very surprising to find that the image persists on the retina even after the object is removed. This is called 'the persistence of vision'. The persistence of vision is approximately one eight of a second.

Even though human eye has the property of accommodation, the muscles cannot be strained beyond a limit and hence if the object is very close to the eye, clear image is not formed on the retina. Thus, there is a minimum distance for the clear vision of an object. This distance is called as 'least distance of distinct vision'. For a normal eye, this distance happens to be 25 cms.