The Human Eye
The human eye is like a camera, the eye has a refracting system containing a converging lens which forms an image of an object in front of the eye on a light-sensitive layer at the back of the eye.
Functions of the Human Eye
The essential parts of the eye and their various functions are:
The thick transparent bulge in the front part of the eye. It serves as a protective covering in front of the lens and allows light into the eye. It also partly focuses on the light entering the eye.
The Aqueous humour is the transparent liquid between the cornea and the lens and the vitreous humour is a jelly-like liquid between the lens and the rest of the eyeball. These liquids serve mainly to keep the eye in its spherical shape.
The Iris is behind the cornea. The iris is the part which gives the colour of the eye. It acts as a stop or diaphram of variable size. It has a tiny opening at its centre called the pupil which regulates the quantity of light entering the eye. The pupil looks black because the inner layer of the eye, the choroid, is black.
The Crystalline lens which focuses the light from the object onto the light-sensitive retina.
The Ciliary muscles support the lens and attach it to the wall of the eye. These muscles by their contraction and expansion, alter the focal length of the lens changing its shape.
The Retina is the light-sensitive area of cells at the back of the eye. The image is formed on the retina which consists of light-sensitive nerve endings which are connected to the optic nerve. The optic nerve conveys the sensation of sight to the brain.
The Yellow spot is the most sensitive spot of the retina. It is here that light entering the eye is usually brought to focus and here the clearest image is formed.
The Blindspot is the point of exit of the optic nerve from the retina. It is insensitive to light.
The Sclerotic layer is the outer covering of the eye.
The optical system of the eye consists of the cornea, the lens, and the aqueous and vitreous humour. They operate to form a real and inverted image of an external object on the retina. The retina transmits the impression created on it by this image to the brain through the optic nerve. The brain then interprets the impressions.
Accommodation is the ability of the eye to alter or adjust the focal length of its lens so as to form clear images of objects at different distances on its retina. This adjustment is brought about through the action of the ciliary muscles.
The eye has an adjusting mechanism for focusing the image of an object on to the retina. It adjusts the focal length of the crystalline lens through the action of the ciliary muscles which control the curvature and hence the focal length of the lens.
Thus through this mechanism both near and far objects can be focused at the retina. This ability of the human eye is known as accommodation.
To see objects close to the eye, the ciliary muscles increase the curvature of the lens, thereby shortening the focal length. The divergent rays from the near object are thus bent more so as to come to a focus on the retina.
Parallel rays from far objects are focused on the retina. Therefore the normal eye is much relaxed and the lens is slim when viewing far objects.
Binocular vision is the overlapping of the two images formed by both eyes which gives an impression of depth and solidity and makes it possible for us to see things in relief. Such a proper perspective of an object is impossible with one eye.
Because our eyes are spaced slightly apart, each views an external object from a slightly different position. Therefore the images of the object falling on the retina are not exactly the same. The right eye sees more of the right side of the object, while the left eye sees more of the left side.
When the two images are combined by the brain, the proper perspective is interpreted.
Binocular vision is possible only if the fields of view of the two eyes overlap. This is not possible in some birds because of the location of the eyes.
Persistence of Vision
When light from an object falls on the retina, it excites it and the clear image formed on the retina leaves a sensation of vision which persists for a short but definite length of time even after the light is removed. This retention of the vision of the image is known as the persistence of vision. If a light flickers at very short intervals of about ten times per second, it appears to the eye as a continuous source of light owing to the persistence of vision.
This phenomenon is made use of in the showing and production of motion pictures, which are a rapid succession of still pictures which are cast on the screen. The persistence of vision makes the still pictures appear to have continuity.
Normal Vision and the Defects of Vision
Near Point and Far Point, The nearest point at which an object can be clearly seen is called the near point. The farthest point at which an object can be clearly seen is called the far point.
For a normal eye, the near point is about 25cm from the eye. This is the limit to which the ciliary muscles can operate, for at this point they are strained to a maximum, and the accommodation is, therefore the greatest. The distance from the near point to the eye is known as the least distance of distinct vision.
For the normal eye, the far point is at infinity. At this point, the ciliary muscles are most relaxed and the lens is slim.
The accommodation is also the least. Additionally, some people whose eyes are not functioning normally say to have some defects of vision. Two of the most common types are the long sight and the short sight. Both defects arise when the lens is unable to accommodate effectively.
Long Sight (Hypermetropia) and its Correction
A long-sighted person can see a far object but cannot see near objects clearly. His near point is farther away than 25 cm, the normal near point. An object at the normal near point N cannot be clearly seen by this eye because the rays from the object are brought to a focus at a certain distance behind the retina. This is caused by the eyeball being too short or the crystalline lens not being sufficiently convergent.
The defect is corrected with the aid of a suitable converging lens placed in front of the eye. The converging lens reduces the divergence of the rays entering the eye from an object just sufficiently to make the rays appear as if they originated from B, the near point of the defective eye. Hence a clear image of the object is formed on the retina.
Comparison of the Human Eye and the Camera
From our previous discussions, we note the following similarities and differences between the human eye and the camera.
Similarities of the Human Eye and the Camera
- The camera includes a light-tight box painted black inside. The human eye is impregnated with black pigments within.
- The pupil in the eye serves the same purpose as the aperture of the camera.
- Both the eye and the camera have a converging lens to focus rays from an external object.
- Both have light-sensitive material on which the rays are focused, the retina in the eye and the film in the camera.
- The iris in the eye serves the same function as the diaphragm in the camera, to regulate the amount of light entering the equipment.
Differences between the Human Eye and the Camera
- The Human eye has the power of accommodation, the ability of the lens to focus points at different distances on the retina by altering the focal length of the lens. Hence the eye lens has a variable focal length but the focal length of the camera lens is fixed.
- The distance between the lens and the retina is fixed in the human eye. In the camera, the distance between the lens and film can be varied.
- The eye can suffer from defects of vision, unlike the camera.
- The eye is a biological organ while the camera is a mechanical device, leading to other differences.