Contents
Physics Topics can be both theoretical and experimental, with scientists using a range of tools and techniques to understand the phenomena they investigate.
What is Range of Vision? What do you Mean by Near Point and Far Point of an Eye?
Structure of Human Eye
Human eye is a perfect natural optical instrument which is more complex and versatile compared to any man-made optical instrument. It may be compared to a camera. If an object be placed in front of a camera, its image is produced in the photographic plate of the camera. Similarly, if an object be placed before a human eye, its image is formed on the screen of the eye i.e., retina. The different parts of the eye are discussed below[Fig.]
Description: The nearly spherical part of the human eye is called the eyeball. It can move in the eye-socket with the help of a few muscles.
A brief description of parts of the eyeball which are relevant for our discussion is given below.
Cornea : It is transparent and its curvature is greater than other positions. Light enters the eye through it. Its refractive index is about 1.33.
Aqueous humour: It is a transparent watery liquid. It acts as the refracting medium of light and occupies the space between the cornea and the eye lens. Its refractive index is about 1.33.
Iris: It is a contractile diaphragm with a circular aperture near the centre. It is made up of two types of muscles. These two muscles contract the iris and control the intensity of the incoming light. The function of the iris is to adjust and admit suitable quantity of light to enter the eye through the pupil.
Pupil : It is a circular aperture at the centre of the iris. Through this, light enters the eye. According to the intensity of the incoming light iris can make the pupil small or large and thus adjust the intensity of the incoming light.
Eye lens: It is a transparent system resembling a double-convex lens, being more convex behind than in front. It is suspended behind the iris by some ligaments. The function of the lens is to form real and inverted images of external objects on the retina. The refractive indices of the material of the different parts of the lens are not equal. The average refractive index of the materials is about 1.45.
Suspensory ligaments: These ligaments confine the eye lens to the right position.
Ciliary Muscle : These muscles adjust the curvature of eye lens and hence the focal length of the eye lens is changed. By adjusting the focal length of the eye lens we help to form the images of the distant objects unkowingly.
Retina: It is a semi-transparent sensitive membrane of nerve fibres forming the inner coating of the eyeball. The optic nerves terminate at this membrane and carry the sensation of sight to be brain as soon as an image of any external object is formed on it.
Yellow spot: In the middle of the retina there is a portion of diameter 2 mm called the yellow spot. This portion of the retina takes the most effective part to understand the colour and details of the object. At the centre of the yellow spot there is a circular portion of diameter 0.3 mm. It is known as fovea centralis. This is the most sensitive part of the retina. If the image of an object is formed on fovea centralis it is seen most distinctly. So the adjusting muscles of the eye always try to cast the image of an object on fovea centralis. To observe by fovea centralis is called direct vision and to observe by other portions of the retina is called indirect vision.
Blind spot: The least sensitive part of the retina is known as blind spot. If the image of an object is formed at this place it is not visible.
Visual axis and optic axis: The line joining the centre of the cornea and that of the lens is called the optic axis of the eye.
The line joining the centre of the lens and the fovea centralis is called the visual axis of the eye. The angle between the optic axis and the visual axis ranges from 5° to 7°.
Function of the eye: Cornea, aqueous humour and the lens form of single convex lens with air on one side and vitreous humour on the other. For a normal eye the focal length of the combination is such that distant objects are focussed on the retina. Rays from external objects enter the eye and undergoes a series of refraction at the cornea, aqueous humour, successive layers of the lens and vitreous humour and finally from a real, inverted and diminished image on the retina. Though the image on the retina is inverted the brain intercepts it as if in the erect position.
With the help of suspensory ligament, ciliary muscle changes the focal length of eye lens in such a way that, image of an object is constructed exactly on retina. So, we can clearly observe an object, irrespective of its position.
Accommodation of the Eye
For a normal eye the focal length of the eye lens is such that objects situated at large distances are focussed on the retina. In this situation, the value of focal length of eye lens is maximum. This focussing is done by the eye by altering the curvature of the lens caused by a change in the tension in the ciliary muscles of the eye lens. When the eye is in a state of full relaxation, distant objects are focussed on the retina. But as the objects move towards the eye, the muscles are put to tension to bring back the
image on the retina which otherwise would have been formed behind the retina.
The involuntary process by which the eye adapts its focal length to see objects at all distances is called accommodation. Acco-modation power of eye is limited. Due to accomodation one can see an object clearly up to a distance 25cm from his/her eye. We can clearly see distant objects like—moon, sun, stars etc. We can also clearly see objects which are very near to us. For example, we can clearly read a book at a small distance from our eyes. But if we bring the book at a distance less than 25 cm from our eyes, then we will not be able to read the book. If we try to read book in this situation for a longer time we will feel pain in our eyes.
Near point: It is the nearest position of the object in front of a normal eye up to which the object can be seen distinctly without accommodation. This shortest distance at which an object can be seen distinctly is called the least distance of distinct vision. For a normal eye this distance is about 25 cm. The process of accommodation cannot function for the normal eye when the object is situated at a distance less than 25 cm from the eye.
Far point: It is the farthest point up to which an object can be distinctly seen without accommodation. For a normal eye the far point is at infinity.
The distance between the near point and the far point is called range of vision. With the help of accommodation a normal eye can see objects situated within this range of vision.
Adaptation of Eye
It has been said earlier that the circular aperture at the centre of the iris is pupil. With the help of some muscles iris can make the pupil small or large and thus adjust the intensity of the incoming light. In presence of powerful light the pupil automatically becomes small and in presence of dim light it becomes large. The increase or decrease in the size of the pupil according to need is called adaptation.
If the light is suddenly extinguished at night in a lighted room we do not see anything in the room momentarily. The reason is that when the room is lighted, the pupil of the eye remains small. Slowly with time the pupil adapts itself to the darkness, becomes larger in size and we are able to discern the furniture and other objects in the room.
Similarly, in a dark place the pupil remains large. So when we suddenly come to a lighted place from a dark one, too much light enters the eye at a time and we are unable to see anything. But after sometime the pupil automatically becomes small and adjusts the amount of incoming light. So we can see everything distinctly. So, if intensity of incident light on eye changes then instantaneously, adaptation of eye takes place and above two incidents are observed.
Persistence of Vision
The retina of the eye continues to bear the effect of light after the stimulus has been taken away. This phenomenon is called the persistence of vision. The interval for which the impressions continue is about \(\frac{1}{10}\) of a second. So if two different incidents occur before our eyes within \(\frac{1}{10}\) of a second, we cannot differen-tiate them. We think that a single incident has happened. This is due to persistence of vision.
For this reason, we cannot sepa-rately visualise the blades of a revolving electric fan. In different times blades are in different positions, but it seems that motion of blades is continuous. For the same reason the glowing end of a splinter or match-stick yields a bright circular track when the former is swung round and not bright points changing their positions in a discontinuous manner.
In this context it needs mention that we can view movies because of persistence of vision. Numerous still pictures of the same thing taken at short intervals, when moved rapidly within \(\frac{1}{10}\) second before the projector, the discontinuous images fuse together to produce an illusion of continuity.
Advantage of Two Eyes
When we look at an object with our two eyes, we see a single object instead of two. On the retina two images are indeed formed by the two eye lenses. But the brain blends the two different images into a single one. As the two eyes are located a little distance apart, we see an object from two different sides. With the right eye we see the front side along with a portion of the right side of the object, similarly with the left eye we see the front side along with a portion of the left side of the object. This perception of the formation of the two images on the retinas of the two eye lenses creates a single perception in the brain.
So we get the three-dimensional idea of the object. The perception of viewing a three-dimensional image of an object with the help of our two eyes is called binocular or stereoscopic vision. We can estimate the distance of the different objects from our eyes and understand actual positions of an object with the help of our two eyes. Fitting thread in a needle hole is a difficult task with one eye closed. As it is difficult to visualise the distance of the needle hole and end of the thread from our eye.