Defect Of Vision And Their Correction Pdf
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The day aims to focus attention worldwide on blindness and vision impairment. The Human eye works on the refraction of light through a natural convex lens made up of transparent living material and enables us to see things around us. And the ability to see is called vision, eyesight, or drishti. The human eye consists of the cornea, iris, pupil, ciliary muscles, eye lens, retina, and optical nerve.
The front part of the eye called the cornea is made of a transparent substance and its outer surface is convex in shape. It is through the cornea that the light coming from objects enters the eyes. Just behind the cornea is the iris which is also called coloured diaphragm.
A hole in the middle of the iris is called the pupil. Then behind it is the eye lens which is a convex lens. It is due to the support of ciliary muscles that the eye lens is held in position.
The eye lens is flexible and thus can change its focal length and shape with the help of ciliary muscles. Behind the eye lens is the retina on which the image is formed in the eye. The light rays coming from the object enter the eyes through the pupil and fall on the eye lens.
The eye lens then converges the light rays and produce an image of the object on the retina which is real and inverted. Retina has a large number of light-sensitive cells that can generate electrical signals. After the image is formed on the retina it sends electrical signals to the brain and we have a sensation of image. Also, even though the image formed on the retina is inverted our mind interprets it as erect. So, the eye lens is the convex lens and the retina is the screen of the eye. The function of iris is to adjust the size of the pupil.
If the amount of light entering the eye is less then the pupil expands so that more light can enter the eye and in case the amount of light entering the eye is large then the pupil contracts. The adjustment of the size of the pupil takes some time and this is the reason when we go outside in the sunlight from a dark room we feel glare in our eyes or if we enter a dark room after coming from outside we see things clearly after some time.
The light-sensitive cells in the retina of our eye are of two shapes; rod shape and cone shape. The function of rod-shaped cells is to respond to the brightness of the light. And the function of cone-shaped cells is to make us see colours and distinguish between them. The ability of the eye to focus distant as well as nearby objects clearly on the retina of the eye is called accommodation.
When our eyes see distant objects then the ciliary muscles relax and the focal length is maximum in this position. The eye-lens then converge the parallel rays of light to form an image of the distant object on the retina. When the eye sees the distant object they are said to be unaccommodated.
And when our eyes see nearby objects then the ciliary muscles get stretched and its focal length decreases. Due to this, the converging power of the eye lens increases, and the diverging rays of light coming from objects converge to form an image on the retina. When the eyes see the nearby objects they are said to be accommodated.
The power of accommodation of a normal eye that enables it to see clearly an object is as close as 25 cm and as far as at infinity. The defect of an eye in which it cannot see the distant objects clearly is called myopia. A person with myopia can see nearby objects clearly. Myopia is caused due to:. Due to the high converging of the eye-lens, the image is formed in front of the retina and a person cannot see clearly the distant objects.
In another case, if the eyeball is too long than the retina is at a larger distance from the eye-lens. In this case, also the image is formed in front of the retina even though the eye-lens has correct converging power. Myopia or short-sightedness can be corrected by wearing spectacles containing a concave lens. This is because when a concave lens of suitable power is used for the myopic eye then the concave lens first diverges the parallel rays of light coming from a distant object.
Therefore, first, a virtual image is formed at the far point of the myopic eye. A concave lens is used for a myopic eye so as to decrease the converging power of the eye-lens. The formula for calculating the power of the concave lens to correct myopia is:. Hypermetropia or long-sightedness is a defect of an eye where a person cannot see nearby objects clearly.
The near-point of the hypermetropic eye is more than 25 cm away. This defect of the eye is caused due to:. In the case of hypermetropia, the image of an object is formed behind the retina and therefore, a person cannot see clearly nearby objects.
The near-point of an eye having hypermetropia is more than 25 cm. The condition of hypermetropia can be corrected by putting a convex lens in front of the eye. This is because when a convex lens of suitable power is placed in front of the hypermetropic eyes, then the convex lens first converges the diverging rays of light coming from a nearby object at the near point of the eye at which the virtual image of the nearby object is formed.
A convex lens is used for hypermetropia so as to increase the converging power of the eye-lens. The formula for calculating power of convex lens to correct hypermetropia is:. In this formula, object distance that is u is normal near the point of the eye 25 cm. This defect of vision usually happens in old age when ciliary muscles become weak and can no longer adjust the eye-lens.
The muscles become inflexible in this condition and cannot see nearby objects clearly. The near-point of an old person having presbyopia is much more than 25 cm. Presbyopia can be corrected by wearing spectacles having a convex lens. Another point to be noted is that a person can have both myopia and hypermetropia.
In such a condition, spectacles having a bifocal lens are worn. The upper part of the bifocal lens is concave and the lower part consists of a convex lens. A yet another defect of the eye which usually comes in old age is the cataract. The medical condition in which the lens of the eye of a person becomes progressively cloudy resulting in blurred vision.
It develops when the eye-lens of a person becomes cloudy due to the formation of a membrane over it. It decreases the vision of the eye gradually and can lead to a total loss of vision of the eye. It can be restored after getting surgery. The opaque lens is removed and an artificial lens is inserted in its place via operation. This defect cannot be corrected by any type of spectacle lenses. To meet our Jury, click here.
Human Eye and Colourful World
Human Eye: working of human eye, Persistence of vision, Power of accommodation of human eye, Defects of vision. The Human Eye: It is a natural optical instrument which is used to see the objects by human beings. It is like a camera which has a lens and screen system. Structure of the Human Eye. The human eye is roughly spherical in shape with a diameter of about 2.
Question 1 What is myopia? State the two causes of myopia? Question 5 What are the two common defects of vision? Question 6 What type of spectacles should be worn by a person having the defects of myopia and hypermetropia? Question 7 What is presbyopia? Write two causes of this defects?
It is an optical instrument just like a photographic camera. It forms the real image of the object on retina of the eye. For normal eye, the range of vision is from 25 cm to infinity. Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea. The crystalline lens merely provides the finer adjustment of focal length required to focus objects at different distances on the retina. We find a structure called iris behind the cornea. Iris is a dark muscular diaphragm that controls the size of the pupil.
Defect Of Vision And Their Correction
Below are given the nature of the defect, its causes and corrective measures Nearsightedness , also called myopia is a common name for impaired vision in which a person sees near objects clearly while distant objects appear blurred. In such a defective eye, the image of a distant object is formed in front of the retina and not at the retina itself. Consequently, a nearsighted person cannot focus clearly on an object farther away than the far point for the defective eye. This may arise due to either.
The day aims to focus attention worldwide on blindness and vision impairment. The Human eye works on the refraction of light through a natural convex lens made up of transparent living material and enables us to see things around us. And the ability to see is called vision, eyesight, or drishti. The human eye consists of the cornea, iris, pupil, ciliary muscles, eye lens, retina, and optical nerve. The front part of the eye called the cornea is made of a transparent substance and its outer surface is convex in shape.
There are many eye related problems and defects of the eye, the main few are briefly discussed below:. Defects of the eye:. Myopia: nearsightedness This is a defect of vision in which far objects appear blurred but near objects are seen clearly. Hyperopia: farsightedness This is a defect of vision in which there is difficulty with near vision but far objects can be seen easily.
Also, the eye doctor checks your eyes for diseases that could lead to vision loss. Vision Correction. You probably know people who need. Defects of Vision and their Correction Nearsightedness. Below are given the nature of the defect, its causes and corrective measures Nearsightednessalso called myopia is a common name for impaired vision in which a person sees near objects clearly while distant objects appear blurred.
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Мысли Стратмора судорожно метались в поисках решения. Всегда есть какой-то выход.
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Глаза ее были полны слез. - Сьюзан. По ее щеке скатилась слеза. - Что с тобой? - в голосе Стратмора слышалась мольба.
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