Working of the human Eye and range of vision.
• Working of the human Eye
We now understand that light coming from an object (either reflected or emitted) enters the eye through the cornea and pupil. The eye lens converges these light rays to form a real, inverted and diminished image of the object on the retina.The light sensitive cells of the retina get activated when light falls on them; they then generate electrical signals. These electric signals are sent to the brain by the optic nerves. The brain interprets the electrical signals in such a way that we see an image which is erect and of the same size as the object.
The image, formed on the retina of eye, does not fade away instantaneously: 1e, impression remains on the retina for about 1/16th of a second even-after the removal of the object. This (brief) continuence, of the sensation of vision, is called persistence of vision. It is because of this persistence of vision that when still images of a moving object are flashed on the eye, at a rate faster than 16 times per second, the eye perceives this object as moving.
The movies, or the T.V. programmes, that we see, are actually made up of a number of separate still pictures in proper sequence. It is the sequence of still pictures, taken by a movie camera, that is projected on the screen at a rate of about 24 images, or more per second. The successive impression of the images on the eye retina appear to blend, or merge, smoothly into one another. We, therefore, see' a moving picture. We make use of this principle in cinematography, or motion-picture projection.
•Range of Vision
Our eyes are such a wonderful optical instrument that they can see distant as wel as nearby objects with almost the same clarity. This becomes possible because or the ability of our eye lens to adjust its shape (curvature) and focal length with the help of the ciliary muscles. When these muscles are relaxed, the focal length is about 2.5 cm and objects at infinity are in sharp focus on the retina.
When an object is brought closer to the eye, the focal length of eye lens becomes shorter. The eye does this because the lens-image distance, for the eye, has to remain constant and equal to (nearly) the size of the eye ball. This special property of the human eye is called its power oft accommodation.
The minimum distance, at which objects can be seen most distinctly (without strain), is called the least distance of distinct vision, or near point, of the eye. For a young adult with normal eyes, this normal near point distance equals (nearly) 25 cm. This distance increases with age due to decreasing effectiveness of ciliary muscles.
The farthest point, up to which the eye can see objects clearly, is called the far point of the eye. It is at infinity (very very far away) for a normal eye.
The distance, between the near point and far point of a normal eye, is called its range of vision. It, thus, varies from (nearly) 25 cm to infinity for the normal eye.
