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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.
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.
Our eyes are such a wonderful optical instrument that they can see distant as well as nearby objects with almost the same clarity.
This becomes possible because of 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 of 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.
Near Point in Human Eyes
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.
Try to read a printed page of your book/newspaper by holding it very close to your eyes.
What do you observe? Are you able to read the page?
The resulting image of the page is somewhat blurred and you feel a strain in your eyes while trying to read it.
Now move the book slightly away from your eyes.
To read this page comfortably and without strain, you have to hold it at a minimum distance (near point of your eye).
The minimum distance, at which you read this page distinctly without strain, is the near point of your eye. Now, try to find out the near point of your friend's eyes. You may use a scale to measure the distance of printed page from the eye in each case.
We now know that a normal eye can see objects over a wide range of distances varying from (nearly) 25 cm to infinity. Sometimes inspite of all precautions and proactive actions, our eyes may develop some defects. These can be due to a variety of reasons. In such a case the eye may not be able to see objects clearly over its normal wide range of distances.
The eye may have one, or both of the following two (common) optical defects:
Myopia (or near sightedness)
Hypermetropia (or far sightedness)
1. Myopia (or near sightedness)
Myopia is a defect of the eye due to which the eye is not able to see distant objects clearly.
In such an eye, the light rays from a distant object, arriving at the eye lens, get converged at a point in front of the retina.
This defect may arise due to either:
(i) excessive curvature of the cornea
(ii) elongation of the eyeball
This defect can be corrected by using a concave (diverging) lens of appropriate focal length.
2. Hypermetropia (or far sightedness)
We may have seen some middle aged people holding a book somewhat away from their eyes to read it properly. This is because they are suffering from hypermetropia.
In this case the image of a nearby object gets formed behind the retina.
This defect arises due to either:
(i) the focal length of eye lens becoming too large
(ii) the eyeball getting shortened
This defect can be corrected using a convex (converging) lens of appropriate focal length.
Sometimes during old age, the eye lens of some people, becomes hazy or even opaque. This happens because of the development of a membrane over it.
When this happens, the person is said to have cataract.
This leads to decrease, or loss, of vision of the eye. It is possible to restore vision in such cases.